Stable and transient expression of chimeric peroxisomal membrane proteins induces an independent "zippering" of peroxisomes and an endoplasmic reticulum subdomain

Peroxisomal ascorbate peroxidase (APX) (EC 1.11.1.11) was shown recently to sort through a subdomain of the ER (peroxisomal endoplasmic reticulum; pER), and in certain cases, alter the distribution and/or morphology of peroxisomes and pER when overexpressed transiently in Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cells. Our goal was to gain insight into the dynamics of peroxisomal membrane protein sorting by characterizing the structure and formation of reorganized peroxisomes and pER. Specifically, we test directly the hypothesis that the observed phenomenon is due to the oligomerization of cytosol-facing, membrane-bound polypeptides. a process referred to as membrane "zippering". Results from differential detergent permeabilization experiments confirmed that peroxisomal APX is a C-terminal "tail-anchored" (Cmatrix-Ncytosol) membrane protein with a majority of the polypeptide facing the cytosol. Transient expression of several APX chimeras whose passenger polypeptides can form dimers or trimers resulted in the progressive formation of "globular" peroxisomes and circular pER membranes. Stable expression of the trimer-capable fusion protein yielded suspension cultures that reproducibly maintained a high degree of peroxisomal globules but relatively few detectable pER membranes. Electron micrographs revealed that the globules consisted of numerous individual peroxisomes, seemingly in direct contact with other peroxisomes and/or mitochondria. These peroxisomal clusters or aggregates were not observed in cells transiently expressing monomeric versions of APX. These findings indicate that the progressive, independent "zippering" of peroxisomes and pER is due to the post-sorting oligomerization of monomeric, cytosol-facing polypeptides that are integrally inserted into the membranes of "like" organelles. The dynamics of this process are discussed, especially with respect to the involvement of the microtubule cytoskeleton.

How are peroxisomes formed? The role of the endoplasmic reticulum and peroxins

Recent data from studies of peroxisome assembly and the subcellular sorting of peroxisomal matrix and membrane proteins have led to an expansion of the 'growth and division' and 'endoplasmic reticulum-vesiculation' models of peroxisome biogenesis into a more flexible, unified model. Within this context, we discuss the proposed role for the endoplasmic reticulum in the formation of preperoxisomes and the potential for 15 Arabidopsis peroxin homologs to function in the biogenesis of peroxisomes in plant cells.

Identification of porin-like polypeptide(s) in the boundary membrane of oilseed glyoxysomes

A 36-kDa polypeptide of unknown function was identified by us in the boundary membrane fraction of cucumber seedling glyoxysomes. Evidence is presented in this study that this 36-kDa polypeptide is a glyoxysomal membrane porin. A sequence of 24 amino acid residues derived from a CNBr-cleaved fragment of the 36-kDa polypeptide revealed 72% to 95% identities with sequences in mitochondrial or non-green plastid porins of several different plant species. Immunological evidence indicated that the 36-kDa (and possibly a 34-kDa polypeptide) was a porin(s). Antiserum raised against a potato tuber mitochondrial porin recognized on immunoblots 34-kDa and 36-kDa polypeptides in detergent-solubilized membrane fractions of cucumber seedling glyoxysomes and mitochondria, and in similar glyoxysomal fractions of cotton, castor bean, and sunflower seedlings. The 36-kDa polypeptide seems to be a constitutive component because it was detected also in membrane protein fractions derived from cucumber leaf-type peroxisomes. Compelling evidence that one or both of these polypeptides were authentic glyoxysomal membrane porins was obtained from electron microscopic immunogold analyses. Antiporin IgGs recognized antigen(s) in outer membranes of glyoxysomes and mitochondria. Taken together, the data indicate that membranes of cucumber (and other oilseed) glyoxysomes, leaf-type peroxisomes, and mitochondria possess similar molecular mass porin polypeptide(s) (34 and 36 kDa) with overlapping immunological and amino acid sequence similarities.

The sorting signals for peroxisomal membrane-bound ascorbate peroxidase are within its C-terminal tail

Peroxisomal ascorbate peroxidase (APX) is a carboxyl tail-anchored, type II (N(cytosol)-C(matrix)) integral membrane protein that functions in the regeneration of NAD(+) in glyoxysomes of germinated oilseeds and protection of peroxisomes in other organisms from toxic H(2)O(2). Recently we showed that cottonseed peroxisomal APX was sorted post-translationally from the cytosol to peroxisomes via a novel reticular/circular membranous network that was interpreted to be a subdomain of the endoplasmic reticulum (ER), named peroxisomal ER (pER). Here we report on the molecular signals responsible for sorting peroxisomal APX. Deletions or site-specific substitutions of certain amino acid residues within the hydrophilic C-terminal-most eight-amino acid residues (includes a positively charged domain found in most peroxisomal integral membrane-destined proteins) abolished sorting of peroxisomal APX to peroxisomes via pER. However, the C-terminal tail was not sufficient for sorting chloramphenicol acetyltransferase to peroxisomes via pER, whereas the peptide plus most of the immediately adjacent 21-amino acid transmembrane domain (TMD) of peroxisomal APX was sufficient for sorting. Replacement of the peroxisomal APX TMD with an artificial TMD (devoid of putative sorting sequences) plus the peroxisomal APX C-terminal tail also sorted chloramphenicol acetyltransferase to peroxisomes via pER, indicating that the peroxisomal APX TMD does not possess essential sorting information. Instead, the TMD appears to confer the proper context required for the conserved positively charged domain to function within peroxisomal APX as an overlapping pER sorting signal and a membrane peroxisome targeting signal type 2.

Peroxisomal membrane ascorbate peroxidase is sorted to a membranous network that resembles a subdomain of the endoplasmic reticulum

The peroxisomal isoform of ascorbate peroxidase (APX) is a novel membrane isoform that functions in the regeneration of NAD(+) and protection against toxic reactive oxygen species. The intracellular localization and sorting of peroxisomal APX were examined both in vivo and in vitro. Epitope-tagged peroxisomal APX, which was expressed transiently in tobacco BY-2 cells, localized to a reticular/circular network that resembled endoplasmic reticulum (ER; 3,3'-dihexyloxacarbocyanine iodide-stained membranes) and to peroxisomes. The reticular network did not colocalize with other organelle marker proteins, including three ER reticuloplasmins. However, in vitro, peroxisomal APX inserted post-translationally into the ER but not into other purified organelle membranes (including peroxisomal membranes). Insertion into the ER depended on the presence of molecular chaperones and ATP. These results suggest that regions of the ER serve as a possible intermediate in the sorting pathway of peroxisomal APX. Insight into this hypothesis was obtained from in vivo experiments with brefeldin A (BFA), a toxin that blocks vesicle-mediated protein export from ER. A transiently expressed chloramphenicol acetyltransferase-peroxisomal APX (CAT-pAPX) fusion protein accumulated only in the reticular/circular network in BFA-treated cells; after subsequent removal of BFA from these cells, the CAT-pAPX was distributed to preexisting peroxisomes. Thus, plant peroxisomal APX, a representative enzymatic peroxisomal membrane protein, is sorted to peroxisomes through an indirect pathway involving a preperoxisomal compartment with characteristics of a distinct subdomain of the ER, possibly a peroxisomal ER subdomain.

Differential subcellular localization of endogenous and transfected soluble epoxide hydrolase in mammalian cells: evidence for isozyme variants

Endogenous, constitutive soluble epoxide hydrolase in mice 3T3 cells was localized via immunofluorescence microscopy exclusively in peroxisomes, whereas transiently expressed mouse soluble epoxide hydrolase (from clofibrate-treated liver) accumulated only in the cytosol of 3T3 and HeLa cells. When the C-terminal lie of mouse soluble epoxide hydrolase was mutated to generate a prototypic putative type 1 PTS (-SKI to -SKL), the enzyme targeted to peroxisomes. The possibility that soluble epoxide hydrolase-SKI was sorted slowly to peroxiosmes from the cytosol was examined by stably expressing rat soluble epoxide hydrolase-SKI appended to the green fluorescent protein. Green fluorescent protein soluble epoxide hydrolase-SKI was strictly cytosolic, indicating that -SKI was not a temporally inefficient putative type 1 PTS. Import of soluble epoxide hydrolase-SKI into peroxisomes in plant cells revealed that the context of -SKI on soluble epoxide hydrolase was targeting permissible. These results show that the C-terminal -SKI is a non-functional putative type 1 PTS on soluble epoxide hydrolase and suggest the existence of distinct cytosolic and peroxisomal targeting variants of soluble epoxide hydrolase in mouse and rat.

Mutational analyses of a type 2 peroxisomal targeting signal that is capable of directing oligomeric protein import into tobacco BY-2 glyoxysomes

In this study of the type 2 peroxisomal targeting signal (PTS2) pathway, we examined the apparent discontinuity and conservation of residues within the PTS2 nonapeptide and demonstrated that this topogenic signal is capable of directing heteromultimeric protein import in plant cells. Based on cumulative data showing that at least 26 unique, putative PTS2 nonapeptides occur within 12 diverse peroxisomal-destined proteins, the current (-R/K-L/V/I-X5-H/Q-L/A-) as well as the original (-R-L-X5-H/Q-L-) PTS2 motif appear to be oversimplified. To assess the functionality of residues within the motif, rat liver thiolase (rthio) and various chimeric chloramphenicol acetyltransferase (CAT) proteins were expressed transiently in suspension-cultured tobacco (Nicotiana tabaccum L.) cv Bright Yellow cells (BY-2), and their subcellular location was determined by immunofluoresence microscopy. Hemagglutinin (HA)-epitope-tagged-CAT subunits, lacking a PTS2 (CAT-HA), were 'piggybacked' into glyoxysomes by PTS2-bearing CAT subunits (rthio-CAT), whereas signal-depleted CAT-HA subunits that were modified to prevent oligomerization did not import into glyoxysomes. These results provided direct evidence that signal-depleted subunits imported into peroxisomes were targeted to the organelle as oligomers (heteromers) by a PTS2. Mutational analysis of residues within PTS2 nonapeptides revealed that a number of amino acid substitutions were capable of maintaining targeting function. Furthermore, functionality of residues within the PTS2 nonapeptide did not appear to require a context-specific environment conferred by adjacent residues. These results collectively suggest that the functional PTS2 is not solely defined as a sequence-specific motif, i.e. -R/K-X6-H/Q-A/L/F-, but defined also by its structural motif that is dependent upon the physiochemical properties of residues within the nonapeptide.

Diverse amino acid residues function within the type 1 peroxisomal targeting signal. Implications for the role of accessory residues upstream of the type 1 peroxisomal targeting signal

The purpose of this study was to determine whether the plant type 1 peroxisomal targeting signal (PTS1) utilizes amino acid residues that do not strictly adhere to the serine-lysine-leucine (SKL) motif (small-basic-hydrophobic residues). Selected residues were appended to the C terminus of chloramphenicol acetyltransferase (CAT) and were tested for their ability to target CAT fusion proteins to glyoxysomes in tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 suspension-cultured cells. CAT was redirected from the cytosol into glyoxysomes by a wide range of residues, i.e. A/C/G/S/T-H/K/ L/N/R-I/L/M/Y. Although L and N at the -2 position (-SLL, -ANL) do not conform to the SKL motif, both functioned, but in a temporally less-efficient manner. Other SKL divergent residues, however, did not target CAT to glyoxysomes, i.e. F or P at the -3 position (-FKL, -PKL), S or T at the -2 position (-SSI, STL), or D at the -1 position (-SKD). The targeting inefficiency of CAT-ANL could be ameliorated when K was included at the -4 position (-KANL). In summary, the plant PTS1 mostly conforms to the SKL motif. For those PTS1s that possess nonconforming residue(s), other residues upstream of the PTS1 appear to function as accessory sequences that enhance the temporal efficiency of peroxisomal targeting.

Identification of the peroxisomal targeting signal for cottonseed catalase

Catalase is a ubiquitous peroxisomal matrix enzyme, yet the molecular targeting signal(s) for sorting it in plant cells has not been defined. The most common peroxisome targeting signal (PTS) is a C-terminal tripeptide composed of a conserved SKL motif (type 1 PTS). The PTS for cottonseed catalase (Ccat) was elucidated in this study from immunofluorescence microscopic analyses of tobacco BY-2 suspension cells serving as an in vivo import system. To distinguish biolistically introduced Ccat from endogenous tobacco catalase, Ccat was hemagglutinin (HA)epitope-tagged at its N-terminus. Bombardment with HA-Ccat resulted in the import of Ccat into glyoxysomes, the specialized type of peroxisome in BY-2 cells. The C-terminal tripeptide of Ccat, PSI, is necessary for import. Evidence for this were mislocalizations to the cytosol of PSI-truncated Ccat and AGV-substituted (for PSI) Ccat. PSI-COOH, however, was not sufficient to re-route chloramphenicol acetyltransferase (CAT) from the cytosol to glyoxysomes, whereas the Ccat tetrapeptide RPSI-COOH was sufficient. Surprisingly, substitution of K (common at the fourth position in other plant catalases) for the R (CAT-KPSI) decreased import efficiency. However, substitution of K did not affect import, when additional upstream residues in Ccat were included (e.g. CAT-NVKPSI). Other evidence for the importance of upstream residues comprised abolishment of Ccat import due to substitutions with non-conserved residues (e.g. -AGVNVRPSI for -SRLNVRPSI). These data indicate that Ccat is sorted to plant peroxisomes by a degenerate type 1 PTS (PSI-COOH) whose residues are functionally dependent on a strict context of adjacent C-terminal amino acid residues.

The plant 73 kDa peroxisomal membrane protein (PMP73) is immunorelated to molecular chaperones

We previously showed via electron microscopic immunocytochemistry that a 73 kDa polypeptide was an authentic peroxisomal membrane protein (PMP73) integrated exclusively into the boundary membrane of glyoxysomes in cucumber seedlings. In this paper we test the hypothesis that this PMP73 is a member of the heat-shock 70 protein (Hsp70) family by comparing amino acid sequences of cyanogen bromide (CNBr)-cleaved polypeptide fragments, immunoreactivities on protein blots, and microscopic immunofluorescence within suspension-cultured BY-2 tobacco cells. A sequence of eight amino acids (DAVGPEIQ) in PMP73 showed a high degree of similarity (up to 88%) with sequences in the same carboxy-terminal region of four plant Hsp70 proteins. IgGs affinity purified to PMP73 recognized on blots a membrane-bound Hsp72 (in pea cotyledon microsomes) and a cucumber PMP61, the latter shown by CNBr cleavage to be a distinct, but immunorelated polypeptide to PMP73. Conversely, IgGs specific for tomato Hsc70 (C-terminal half) recognized cucumber PMP73, and IgGs specific for cucumber DnaJ homologue (entire protein) recognized cucumber PMP61. In BY-2 cells, cucumber PMP73-specific IgGs localized only to peroxisomes. Antibodies raised against portions of tomato Hsc70 also localized to the BY-2 peroxisomes (as well as to cytosolic proteins). Collectively, the data show that authentic cucumber PMPs73 and 61 are immunorelated to each another, and that both exhibit selective immunoreactivity to IgGs from two classes of molecular chaperones, namely Hsp70 proteins and plant DnaJ homologues. They appear to be unique membrane-bound chaperones that likely function as part of the peroxisomal protein translocation machinery.

Oilseed isocitrate lyases lacking their essential type 1 peroxisomal targeting signal are piggybacked to glyoxysomes

Isocitrate lyase (IL) is an essential enzyme in the glyoxylate cycle, which is a pathway involved in the mobilization of stored lipids during postgerminative growth of oil-rich seedlings. We determined experimentally the necessary and sufficient peroxisome targeting signals (PTSs) for cottonseed, oilseed rape, and castor bean ILs in a well-characterized in vivo import system, namely, suspension-cultured tobacco (Bright Yellow) BY-2 cells. Results were obtained by comparing immunofluorescence localizations of wild-type and C-terminal-truncated proteins transiently expressed from cDNAs introduced by microprojectile bombardment. The tripeptides ARM-COOH (on cottonseed and castor bean ILs) and SRM-COOH (on oilseed rape IL) were necessary for targeting and actual import of these ILs into glyoxysomes, and ARM-COOH was sufficient for redirecting chloramphenicol acetyltransferase (CAT) from the cytosol into the glyoxysomes. Surprisingly, IL and CAT subunits without these tripeptides were still acquired by glyoxysomes, but only when wild-type IL or CAT-SKL subunits, respectively, were simultaneously expressed in the cells. These results reveal that targeting signal-depleted subunits are being piggybacked as multimers to glyoxysomes by association with subunits possessing a PTS1. Targeted multimers are then translocated through membrane pores or channels to the matrix as oligomers or as subunits before reoligomerization in the matrix.

Rat liver catalase is sorted to peroxisomes by its C-terminal tripeptide Ala-Asn-Leu, not by the internal Ser-Lys-Leu motif

The molecular signal for targeting catalases to peroxisomes has not been defined. In this study, a plant in vivo import system (tobacco BY-2 suspension culture cells) was used to test the current postulate that the peroxisome targeting signal (PTS) for mammalian catalases is the internal Ser-Lys-Leu (SKL) motif found approximately eight amino acid residues from the C-terminus. Elucidation of the catalase PTS has been hampered previously by the ubiquitous presence of catalase in peroxisomes. The current study was possible because antibodies to mammalian catalases did not recognize endogenous, tobacco peroxisome catalase. Rat and mouse liver catalases (Rcat and Mcat), with an internal Ser-His-Ile (SHI) and Ser-His-Met (SHM), respectively, and both with a C-terminal Ala-Asn-Leu (ANL), were expressed transiently in BY-2 cells and targeted to the peroxisomes. Sorting was demonstrated by double-label immunofluorescence colocalization of these catalases with tobacco catalase. Peroxisome targeting of Rcat was abolished as expected when the internal SHI residues were removed by deletion of three C-terminal portions (28, 16, or 11 residues). Surprisingly, peroxisome targeting was still abolished when SHI (or SHL produced by site-directed mutagenesis) were at the extreme C-terminus as a consequence of deleting eight residues. However, when SHL was at the C-terminus in full-sized Rcat via a mutation of ANL-COOH, the enzyme sorted to peroxisomes indicating that the position of the PTS is significant in Rcat. The importance of the internal context of the SHI (or SHL) was examined further by changing ANL-COOH to a non-SKL motif, AGS-COOH. This Rcat did not sort to the peroxisomes, nor did Rcat with its ANL-COOH deleted; these data indicated the necessity of the C-terminal tripeptide. Sufficiency of ANL was demonstrated when chloramphenicol acetyltransferase with an appended ANL-COOH was redirected from the cytosol to peroxisomes. Collectively, these results do not support the internal PTS hypothesis, but indicate that a type 1 PTS slightly divergent from the typical SKL motif serves as the necessary and sufficient PTS for rat liver and probably other eukaryotic catalases.

Ascorbate peroxidase. A prominent membrane protein in oilseed glyoxysomes

The glyoxysomes of growing oilseed seedlings produce H2O2, a reactive oxygen species, during the beta-oxidation of lipids stored in the cotyledons. An expression library of dark-grown cotton (Gossypium hirsutm L.) cotyledons was screened with antibodies that recognized a 31-kD glyoxysomal membrane polypeptide. A full-length cDNA clone (1258 bp) was isolated that encodes a 32-kD subunit of ascorbate peroxidase (APX) with a single, putative membrane-spanning region near the C-terminal end of the polypeptide. Internal amino acid sequence analysis of the cotton 31-kD polypeptide verified that this clone encoded this protein. This enzyme, designated gmAPX, was immunocytochemically and enzymatically localized to the glyoxysomal membrane in cotton cotyledons. The activity of monodehydroascorbate reductase, a protein that reduces monodehydroascorbate to ascorbate with NADH, also was detected in these membranes. The co-localization of gmAPX and monodehydroascorbate reductase within the glyoxysomal membrane likely reflects an essential pathway for scavenging reactive oxygen species and also provides a mechanism to regenerate NAD+ for the continued operation of the glyoxylate cycle and beta-oxidation of fatty acids. Immunological cross-reactivity of 30- to 32-kD proteins in glyoxysomal membranes of cucumber, sunflower, castor bean, and cotton indicate that gmAPX is common among oilseed species.

Development and application of an in vivo plant peroxisome import system

The purposes of this study are to develop an in vivo cell system that is suitable for the immunofluorescent detection of transiently expressed proteins targeted to plant peroxisomes and to determine whether a C-terminal serine-lysine-leucine (SKL) tripeptide, a consensus-targeting signal for mammalian peroxisomes, also targets proteins to plant peroxisomes. Protoplasts from mesophyll cells and from suspension-cultured cells initially were examined for their potential as an in vivo import system. Several were found suitable, but based on a combination of criteria, suspension-cultured tobacco (Nicotiana tabacum L. cv Bright Yellow 2) cells (TBY-2) were chosen. The tobacco cell extracts had catalase activity, and two polypeptides of approximately 55 and 57 kD specifically were detected on immunoblots with anti-cottonseed catalase immunoglobulins G as the probe. Indirect immunofluorescence microscopy with these immunoglobulins G revealed a punctate labeling pattern indicative of endogenous catalase localization within putative TBY-2 peroxisomes. The cells did not have to be completely converted to protoplasts for optimal microscopy; treatment with 0.1% (w/v) pectolyase for 2 h was sufficient. Microprojectile bombardment proved superior for transient transformation of the TBY-2 cells with plasmids encoding beta-glucuronidase, or chloramphenicol acetyltransferase (CAT), or CAT with an added C-terminal tripeptide (CAT-SKL). C-terminal SKL is a consensus, type 1, peroxisome targeting signal. Double indirect immunofluorescent labeling showed that CAT-SKL co-localized with endogenous catalase. Non-punctate, diffuse localization of CAT without SKL provided direct evidence that the C-terminal SKL tripeptide was necessary and sufficient for targeting of CAT to plant peroxisomes. These data demonstrate the effectiveness of this peroxisome targeting signal for plant cells.

Nucleotide and deduced amino acid sequence of a putative higher molecular weight precursor for catalase in sunflower cotyledons

A cDNA from a sunflower (Helianthus annuus L.) library encoded a 56.8 kDa catalase peptide. N-terminal sequence comparisons to peroxisomal higher molecular weight precursors revealed conserved amino acid motifs around the (putative) cleavage sites. These findings suggest that the 55 kDa catalase in sunflower cotyledons is synthesized at a higher molecular weight.

Identification and immunochemical characterization of a family of peroxisome membrane proteins (PMPs) in oilseed glyoxysomes

Prior to this study the only antibodies available for characterizing peroxisome membrane proteins (PMPs) in plants were the antibodies raised against membranes isolated from castor bean endosperm glyoxysomes by Halpin et al. (Planta 179, 331-339 (1989)). We raised antibodies to four different nondenatured PMP complexes solubilized in 0.63 M aminocaproate/1% dodecylmaltoside from alkaline carbonate-washed, cucumber cotyledon glyoxysome membranes. The four complexes, approximately 290/270, 148, 128 and 67 kDa, were excised from 5 to 10% nondenaturing gradient gels, passively eluted from their homogenized gel slice, concentrated, then injected subcutaneously into rabbits. SDS-PAGE (10-15% gradient) of the total detergent-solubilized PMPs revealed six prominent membrane polypeptides: 73, 61, 52, 36, 30, and 22 kDa. The SDS-PMP composition of each nondenatured antigen was: PMP290/270-52, 30, 28 kDa; PMP148-30, 28, 26, 23, 22 kDa; PMP 128-73, 66, 36, 30, 23 kDa; PMP67-34, 30 kDa. These data indicated that several prominent as well as several minor polypeptides were common components of the PMP complexes. Three of the four antisera to the complexes were polyspecific, recognizing several of these common SDS polypeptides, whereas the fourth antiserum, anti-PMP67, was monospecific for PMP30. Cross-reactivities were evident with each antiserum to several of these SDS PMPs from castor bean, cotton and sunflower. Affinity-purified anti-PMP30 and anti-PMP73 antibodies specifically bound to the boundary membrane of cucumber glyoxysomes in cells examined by indirect, postembedment (LR White), immunocytochemistry. These, and the family of other antibodies produced in this study, provide specific molecular probes essential for elucidating biogenesis and discovering function(s) of the integral membrane proteins in oilseed glyoxysomes.

Conservative amino acid substitutions of the C-terminal tripeptide (Ala-Arg-Met) on cottonseed isocitrate lyase preserve import in vivo into mammalian cell peroxisomes

The purpose of this research was twofold, a) to directly demonstrate import in vivo of a native plant peroxisomal protein into peroxisomes of transiently transfected mammalian cells, and b) to identify the targeting signal and amino acid substitutions thereof which preserve translocation of this plant protein into these peroxisomes. The protein selected for study was cottonseed isocitrate lyase (ICL), a glyoxylate cycle enzyme which participates in storage oil mobilization in oilseed cotyledons. Cultured mammalian cells were selected as the import system because of previous success by others with transient transfections and import of heterologous (not plant, however) proteins, and because neither a plant in vitro or transient in vivo import system was established. Optimized transient transfections of cultured CV-1 monkey kidney, mouse L, HeLa, and CHO cells resulted in punctate, anticottonseed-ICL-dependent immunofluorescent patterns. Colocalization in a CVH Px110 cell line of ICL with either endogenous catalase or with stably expressed CAT-PMP20/AKL (chloramphenicol acetyltransferase with a C-terminal-appended 12 amino acids ending with Ala-Lys-Leu) demonstrated targeting of ICL to peroxisomes. Direct evidence for translocation of ICL into CHO cell peroxisomes was obtained from digitonin permeabilization experiments. The necessity of the C-terminal tetrapeptide, KARM-COOH, was demonstrated in CHO and CV-1 cells when removal of this tetrapetide (leaving ICL-VVA-COOH) abolished import into peroxisomes. This result is in general agreement with Olsen et al. (The Plant Cell 5, 941-952 (1993)) who demonstrated that the 37 C-terminal amino acids of oilseed rape ICL were necessary for import in vivo in transgenic plants. The findings of Behari and Baker (J. Biol. Chem. 268, 7315-7322 (1993)), however, indicate that the C-terminal portion of castor bean ICL is dispensible for import in vitro. Single or multiple conservative amino acid substitutions at each position of the C-terminal tripeptide of native cottonseed ICL (S for A, K for R, L for M, SK for AR, SKL for ARM) preserved import of the enzyme in vivo into CHO cell peroxisomes. The demonstrated targeting and translocation of plant ICL and C-terminal modifications thereof into mammalian cell peroxisomes provide important additional evidence for evolutionary conservation of peroxisome import machinery, especially relative to the PTS1 sequence.

Identification of Peroxisome Membrane Proteins (PMPs) in Sunflower (Helianthus annuus L.) Cotyledons and Influence of Light on the PMP Developmental Pattern

Boundary membranes were recovered from glyoxysomes, transition peroxisomes, and leaf-type peroxisomes purified from cotyledons of sunflower (Helianthus annuus L.) at three stages of postgerminative growth. After membranes were washed in 100 mM Na2CO3 (pH 11.5), integral peroxisome membrane proteins (PMPs) were solubilized in buffered aminocaproic acid/dodecyl maltoside (0.63 M/1.5%) and analyzed by nondenaturing and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Six prominent nondenatured PMP complexes and 10 prominent SDS-denatured polypeptides were identified in the membranes of the three types of peroxisomes. A nondenatured complex of approximately 140 kD, composed mainly of 24.5-kD polypeptides, decreased temporally, independently of seedling exposure to white, blue, or red light; only far-red light seemed to prevent its decrease. PMP complexes of approximately 120 and 70 kD, in contrast, were present at all stages and changed in polypeptide content. It remains to be determined whether these data reflect changes within in vivo complexes or within complexes formed following/during detergent solubilization. Conversion of glyoxysomes to leaf-type peroxisomes in white or red light after a 2-d dark period was accompanied by the appearance of three SDS-denatured PMPs: 27.5, 28, and 47 kD. The former two became part of the PMP120 and 70 complexes, as well as part of a new PMP130 complex that also possessed the PMP47. Growth of seedlings in blue or far-red light did not promote the appearance of PMPs 27.5 or 28. Blue light promoted the appearance of PMP47, and far-red light seemed to prevent its appearance. Chlorophyll likely is not the photoreceptor involved in accumulation of PMPs because the PMP composition is distinctly different in seedlings irradiated with red or blue light of comparable fluence rates. Several lines of evidence indicate that the synthesis and acquisition of membrane and all matrix proteins are not coupled. The data provide evidence for a change in PMP composition when sunflower or any other oilseed glyoxysomes are converted to leaf-type peroxisomes and suggest that the change is regulated by both photobiological and temporal mechanisms.

Acquisition of membrane lipids by differentiating glyoxysomes: role of lipid bodies

Glyoxysomes in cotyledons of cotton (Gossypium hirsutum, L.) seedlings enlarge dramatically within 48 h after seed imbibition (Kunce, C.M., R.N. Trelease, and D.C. Doman. 1984. Planta (Berl.). 161:156-164) to effect mobilization of stored cotton-seed oil. We discovered that the membranes of enlarging glyoxysomes at all stages examined contained a large percentage (36-62% by weight) of nonpolar lipid, nearly all of which were triacylglycerols (TAGs) and TAG metabolites. Free fatty acids comprised the largest percentage of these nonpolar lipids. Six uncommon (and as yet unidentified) fatty acids constituted the majority (51%) of both the free fatty acids and the fatty acids in TAGs of glyoxysome membranes; the same six uncommon fatty acids were less than 7% of the acyl constituents in TAGs extracted from cotton-seed storage lipid bodies. TAGs of lipid bodies primarily were composed of palmitic, oleic, and linoleic acids (together 70%). Together, these three major storage fatty acids were less than 10% of both the free fatty acids and fatty acids in TAGs of glyoxysome membranes. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) constituted a major portion of glyoxysome membrane phospholipids (together 61% by weight). Pulse-chase radiolabeling experiments in vivo clearly demonstrated that 14C-PC and 14C-PE were synthesized from 14C-choline and 14C-ethanolamine, respectively, in ER of cotyledons, and then transported to mitochondria; however, these lipids were not transported to enlarging glyoxysomes. The lack of ER involvement in glyoxysome membrane phospholipid synthesis, and the similarities in lipid compositions between lipid bodies and membranes of glyoxysomes, led us to formulate and test a new hypothesis whereby lipid bodies serve as the dynamic source of nonpolar lipids and phospholipids for membrane expansion of enlarging glyoxysomes. In a cell-free system, 3H-triolein (TO) and 3H-PC were indeed transferred from lipid bodies to glyoxysomes. 3H-PC, but not 3H-TO, also was transferred to mitochondria in vitro. The amount of lipid transferred increased linearly with respect to time and amount of acceptor organelle protein, and transfer occurred only when lipid body membrane proteins were associated with the donor lipid bodies. 3H-TO was transferred to and incorporated into glyoxysome membranes, and then hydrolyzed to free fatty acids. 3H-PC was transferred to and incorporated into glyoxysome and mitochondria membranes without subsequent hydrolysis. Our data are inconsistent with the hypothesis that ER contributes membrane lipids to glyoxysomes during postgerminative seedling growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Two genes encode the two subunits of cottonseed catalase

The isolation and sequence of a cDNA encoding a developmentally distinct subunit of cottonseed catalase are presented. A 1.8-kb cDNA was selected from a cDNA library constructed with poly(A)+ RNA isolated from 3-day-old dark-grown cotyledons in which a second subunit (designated SU 2 in an earlier publication) of catalase was predominantly synthesized. The cDNA encodes a 492-amino acid peptide with a calculated Mr of 56,900. The nucleotide sequence is 76% identical to a cDNA encoding another subunit (SU 1) which was predominantly synthesized in 1-day-old-cotyledons. Most of the divergence occurs in the 5' and 3' non-coding regions, and at the third positions of the codons. The deduced amino acid sequence is 92% identical to that of SU 1. Denaturing isoelectric focusing and SDS-PAGE of products transcribed and translated in vitro from these cDNAs revealed that the cDNA selected from the "1-day" library encoded SU 1 and the cDNA selected from the "3-day" library (this paper) encoded SU 2 of catalase. These data and results from Southern blot analyses of genomic DNA indicate that there are two genes encoding catalase subunits in cotton cotyledons, with only one copy of SU 1 and at least two copies of SU 2 in the genome. A peroxisomal targeting signal, e.g., Ser-Lys-Leu, is not located at the C-terminus of either subunit, or within 25 residues of the C-terminus of SU 1, although it occurs at six residues upstream from the C-terminus of SU 2. A possible location of a targeting sequence for catalase and other peroxisomal proteins lacking the C-terminal tripeptide motif is proposed.

Post-Transcriptional Regulation of Catalase Isozyme Expression in Cotton Seeds

We reported previously that expression of the five tetrameric catalase isozymes during postgerminative growth of cotton seedings was a consequence of interactions between two subunits (SU 1 and SU 2) temporally synthesized from two distinct catalase genes. In this study, we focused on the regulation of the expression of these two catalase subunits during the changeover from glyoxysomal to leaf-type peroxisomal metabolism. The steady-state level of glyoxysomal SU 1 protein (present in 12-hour-old seeds) increased through day 3 and then declined linearly through day 6, whereas SU 2 protein (first detected in 24-hour-old seeds) increased continuously through day 6. The time courses for steady-state levels of the mRNAs encoding these two subunits revealed two clearly separated peaks: the first at day 1 (SU 1) and the other at day 4 (SU 2). Accumulation of these mRNAs preceded the accumulation of their corresponding proteins by at least 24 hours, suggesting temporal, pretranslational regulation of synthesis of both subunits. Results from run-on transcriptional assays with isolated nuclei, however, revealed that transcripts encoding both subunits were synthesized together on days 1 through 5. Hence, it appears that the accumulations of SU 1 and SU 2 mRNAs are controlled primarily at the post-transcriptional level, which has not been reported for catalase or any other eukaryotic peroxisomal enzymes. The accumulation of SU 1 mRNA is not light dependent, whereas the accumulation of SU 2 mRNA, which directs synthesis of the predominant subunit comprising the leaf-type peroxisomal isozyme, occurs only after exposure of seedlings to light.

Intracellular localization of phosphatidylcholine and phosphatidylethanolamine synthesis in cotyledons of cotton seedlings

Subfractionation of clarified cotyledon homogenates of cotton (Gossypium hirsutum L.) seedlings on sucrose gradients revealed a single coincident peak of cholinephosphotransferase (EC 2.7.8.2) (CPT) and ethanolaminephosphotransferase (EC 2.7.8.1) (EPT) activities, which equilibrated with the main peak of Antimycin A-insensitive NADH:cytochrome c reductase (CCR) activity. The small percentage of CPT and EPT activities (less than 5% of the total) in glyoxysome-enriched pellets equilibrated with cytochrome c oxidase activity, not with catalase activity. Preincubation of microsomes (containing 83% of total CPT and EPT activities) in 0.2 millimolar MgCl(2) followed by subfractionation on sucrose gradients resulted in peak CPT and EPT activities equilibrating with peak CCR activity at 24% (w/w) sucrose. Preincubation of microsomes with (14)C-CDPcholine (or (14)C-CDPethanolamine) resulted in synthesis and incorporation of (14)C-phosphatidylcholine (PC) (or (14)C-phosphatidylethanolamine, PE) into membranes at the same density. Increasing the Mg(2+) concentration to 2.0 millimolar facilitated binding of ribosomes and caused a concomitant shift in density (to 34% w/w sucrose) of peak CPT, EPT, and CCR activities. Under these conditions, newly synthesized and incorporated (14)C-PC (or PE) was recovered in these membranes. Transmission electron microscopy of this fraction confirmed binding of ribosomes to membranes. Radiolabeling in vivo of cotyledons with [methyl-(14)C] choline chloride or [1,2 ethanolamine-(14)C] ethanolamine hydrochloride resulted in a linear incorporation of radiolabel into PC or PE in a time dependent manner. Subfractionation of homogenates of radiolabeled cotyledons on sucrose gradients showed that membranes sedimenting at 24% (w/w) sucrose (ER) contained the majority of radiolabeled PC and PE with a minor peak at 40% (w/w) sucrose (mitochondria), but no radioactive PC or PE was recovered in glyoxysomes. These results indicate that ER in cotyledons of germinated cotton seedlings is the primary subcellular site of PC and PE synthesis. This is similar to the situation in endosperm tissue but distinctly different from root and hypocotyl tissue where Golgi are a major subcellular site of PC and PE synthesis.

Inhibition of Cottonseed Choline- and Ethanolaminephosphotransferases by Calcium during Postgerminative Growth

Activities of choline- and ethanolaminephosphotransferase (CPT and EPT) were reproducibly high in microsomes from imbibed seeds of cotton (Gossypium hirsutum, L.). Initial studies showed that both activities dramatically declined during postgerminative growth when demand for phosphatidylcholine (PC) and phosphatidylethanolamine (PE) synthesis was high. Addition of CaCl(2) (0.1 millimolar) or aliquots of supernatant fractions (150,000g, 60 minutes) from cotyledons of 48-hour-old seedlings to imbibed-seed microsomes reduced the CPT and EPT activities to levels approximating those found in 48-hour microsomes. Inhibition by supernatants was completely reversed by adding EGTA (1.0 millimolar), but not by boiling the supernatants. EGTA (1.0 or 5.0 millimolar) relieved inhibition in cellular fractions whether it was added to the homogenization media or the assay reaction mixtures. A time course of CPT and EPT activities in cellular fractions prepared with 1.0 millimolar EGTA showed that activities were well developed in imbibed seeds, doubled coincidentally to a peak at 36 hours, then declined during the next 12 hours to levels approximating those in imbibed seeds. Greater than 90% of the CPT and EPT activities were pelletable (150,000g, 60 minutes) at all ages examined. Calcium apparently was artificially released upon homogenization, to a progressively greater extent in older cotyledons, and severely inhibited CPT and EPT activities. This is the only time course of CPT and EPT activities reported for cotyledons of any oilseed; it is substantially different from that in oil-storing endosperm.

Two temporally synthesized charge subunits interact to form the five isoforms of cottonseed (Gossypium hirsutum) catalase

Five charge isoforms of tetrameric catalase were isolated from cotyledons of germinated cotton (Gossypium hirsutum L.) seedlings. Denaturing isoelectric focusing of the individual isoforms in polyacrylamide gels indicated that isoforms A (most anodic) and E (most cathodic) consisted of one subunit of different charge, whereas isoforms B, C and D each consisted of a mixture of these two subunits. Thus the five isoforms apparently were formed through combinations of two subunits in different ratios. Labelling cotyledons in vivo with [35S]methionine at three daily intervals in the dark, and translation in vivo of polyadenylated RNA isolated from cotyledons at the same ages, revealed synthesis of two different subunits. One of the subunits was synthesized in cotyledons at all ages studied (days 1-3), whereas the other subunit was detected only at days 2 and 3. This differential expression of two catalase subunits helped explain previous results from this laboratory showing that the two anodic forms (A and B) found in maturing seeds were supplemented with three cathodic forms (C-E) after the seeds germinated. These subunit data also helped clarify our new findings that proteins of isoforms A, B and C (most active isoforms) accumulated in cotyledons of plants kept in the dark for 3 days, then gradually disappeared during the next several days, whereas isoforms D and E (least active isoforms) remained in the cells. This shift in isoform pattern occurred whether seedlings were kept in the dark or exposed to continuous light after day 3, although exposure to light enhanced this process. These sequential molecular events were responsible for the characteristic developmental changes (rise and fall) in total catalase activity. We believe that the isoform changeover is physiologically related to the changeover in glyoxysome to leaf-type-peroxisome metabolism.

Characterization of a cDNA encoding cottonseed catalase

A 1.7 kb cDNA clone was isolated from our lambda gt11 library constructed from poly(A) RNA of 24-h-old cotyledons. The cDNA encodes a full-length catalase peptide (492 amino acid residues). The calculated molecular mass is 56,800, similar to that determined for purified enzyme (57,000 SDS-PAGE). Among higher plant catalases, this cotton catalase shows the highest amino acid sequence identity (85%) to the subunit of homotetrameric maize CAT 1, a developmental counterpart to the homotetrameric CAT A isoform of cotton seeds. Comparison of sequences from cotton, sweet potato, maize CAT 1, and yeast with bovine catalase revealed that the amino acid residues and regions that are involved in catalytic activity and/or required to maintain basic catalase structure, are highly conserved. The C-terminus region, which has the lowest nucleotide sequence identity between plant and mammalian catalases, does not terminate with a tripeptide, S-K/R/H-L, a putative targeting signal for peroxisomal proteins.

Characterization of a cDNA clone encoding the complete amino acid sequence of cotton isocitrate lyase

A cDNA clone encoding the glyoxysomal enzyme isocitrate lyase (ICL) (EC 4.1.3.1) was isolated from a library prepared from cotton (Gossypium hirsutum L.) cotyledon poly(A)+ RNA. The clone is 1893 basepairs (bp) in length and contains a 1728 bp open reading frame encoding a polypeptide of 576 residues (Mr = 64,741). The deduced amino acid sequence of cotton ICL is 85.2%, 90.3% and 41.1% identical to ICL from rapeseed, castor bean and E. coli, respectively. Cotton ICL has a C-terminal tripeptide of A-R-M which is a putative trafficking signal for peroxisome (glyoxysome) proteins.

Biogenesis of catalase in glyoxysomes and leaf-type peroxisomes of sunflower cotyledons

Eight charge isoforms of catalase (EC 1.11.1.6.) appeared in the peroxisomes of sunflower cotyledons during growth after germination (2.5 days of dark, continuous light thereafter). In the light, when glyoxysomes were transformed to leaf-type peroxisomes, the five more-basic forms (CAT 1 through CAT 5) became more prominent, while amounts of the three more-acidic forms (CAT 6 through CAT 8) decreased considerably. The isoforms CAT 1 through CAT 5 were hybrids of 55- and 59-kDa subunits, whereas CAT 6 through CAT 8 contained 55-kDa subunits exclusively. The catalase translation products changed during the transition of glyoxysomes to leaf-type peroxisomes. Polyadenylated RNA from 2-day-old cotyledons directed synthesis of 56-kDa subunits, whereas 59-kDa subunits predominated after in vitro translation of RNA from 4-day-old cotyledons. Both translation products were processed to lower molecular weight forms in vivo. The 56-kDa translation products were precursors for 55-kDa subunits in glyoxysomes. It could not be decided however, whether the 59-kDa precursors were processed to 56-kDa or 55-kDa subunits, because both subunits of lower molecular weight were present in leaf-type peroxisomes. Some of the 59-kDa precursors escaped proteolytic processing and formed hybrid isoforms (CAT 1 through CAT 5) with mature 55-kDa subunits. This type of isoform formation, i.e., condensation of mature and unprocessed subunits, has not yet been described for other plant catalases. In summary, the results showed that the postgerminative changes in the number and abundance of catalase isoforms resulted from changes in translation (transcription) of catalase precursors and assembly of proteolytically processed and unprocessed subunits into tetramers within peroxisomes acquiring leaf peroxisomal function.

Relationship between Cottonseed Malate Synthase Aggregation Behavior and Suborganellar Location in Glyoxysomes and Endoplasmic Reticulum

Malate synthase (EC 4.1.3.2) (MS), an enzyme unique to the glyoxylate cycle, was studied in cotyledons of dark-grown cotton (Gossypium hirsutum, L.) seedlings. MS has generally been regarded as a peripheral membrane protein in glyoxysomes and believed by some to be synthesized on rough ER. Immunocyto-chemical localization of MS in both in situ and isolated cottonseed glyoxysomes, however, showed that MS was located throughout the matrix of glyoxysomes, not specifically associated with their membranes. Biochemical data also supported matrix localization. Isolated glyoxysomes were diluted in variously-buffered salt solutions (200 millimolar KCl or 100 millimolar K-phosphate) or detergents (0.1% Triton X-100, 10 millimolar deoxycholate, or 1.0% Triton X-114) and centrifuged to pellet membranes. Greater than 70% of the MS was recovered in supernatants after treatment with salt solutions, whereas generally less than 30% was released following detergent treatments. MS in pellets derived from glyoxysomes burst in low ionic strength buffer solutions was aggregated (observed on rate-zonal gradients). MS released following salt treatments was the 20S nonaggregated form indicating that salt solutions either disaggregated (or prevented aggregation of) glyoxysomal MS rather than releasing it from membranes. We confirmed reports by others that MS comigrated with ER (NADH: cytochrome c reductase) in sucrose (20-40% w/w) gradients buffered with 100 millimolar Tricine (pH 7.5) after 3 hours centrifugation. However, cottonseed MS did not comigrate with ER in gradients buffered with 10 millimolar Hepes (pH 7.0) or 20 millimolar K-phosphate (pH 7.2) after 3 hours centrifugation, or after 22 hours centrifugation in Tricine or Hepes. Collectively, our data with cotton seeds indicate that MS is not a peripheral membrane protein, and that the aggregation behavior of MS (in various buffers) very likely has led to misinterpretations of its putative associations with ER and glyoxysomal membranes.

Purification and biosynthesis of cottonseed (Gossypium hirsutum L.) catalase

As part of our research on peroxisome biogenesis, catalase was purified from cotyledons of dark-grown cotton (Gossypium hirsutum L.) seedlings and monospecific antibodies were raised in rabbits. Purified catalase appeared as three distinct electrophoretic forms in non-denaturing gels and as a single protein band (with a subunit Mr of 57,000) on silver-stained SDS/polyacrylamide gels. Western blots of crude extracts and isolated peroxisomes from cotton revealed one immunoreactive polypeptide with the same Mr (57,000) as the purified enzyme, indicating that catalase did not undergo any detectable change in Mr during purification. Synthesis in vitro, directed by polyadenylated RNA isolated from either maturing seeds or cotyledons of dark-grown cotton seedlings, revealed a predominant immunoreactive translation product with a subunit Mr of 57,000 and an additional minor immunoreactive product with a subunit Mr of 64000. Labelling studies in vivo revealed newly synthesized monomers of both the 64000- and 57,000-Mr proteins present in the cytosol and incorporation of both proteins into the peroxisome without proteolytic processing. Within the peroxisome, the 57,000-Mr catalase was found as an 11S tetramer; whereas the 64,000-Mr protein was found as a relatively long-lived 20S aggregate (native Mr approx. 600,000-800,000). The results strongly indicate that the 64,000-Mr protein (catalase?) is not a precursor to the 57,000-Mr catalase and that cotton catalase is translated on cytosolic ribosomes without a cleavable transit or signal sequence.

Regulation of isocitrate lyase gene expression in sunflower

A cDNA sequence that encodes a portion of sunflower (Helianthus annuus L.) seedling isocitrate lyase was selected from a lambda gt11 cDNA library derived from sunflower seedling cotyledon poly(A)(+) messenger RNA. The library was screened for bacteriophage recombinants that expressed antigens which reacted with antisera directed against cotton seed isocitrate lyase. The isolated cDNA hybridized with a 2 kilobase RNA species that was first detectable in maturing sunflower embryos 19 days after flowering and remained at a constant low level through seed desiccation. The prevalence of this transcript in sunflower cotyledons increased by about 10-fold within 2 days after inhibition in darkness, and transcript levels began to decrease by 5 days after imbibtion. During the first 2 days of germination and growth of sunflower seedlings in light, the rate of isocitrate lyase mRNA accumulation was greater than the rate observed during this period in dark-grown seedlings, giving peak levels about 2-fold higher than corresponding levels in dark-grown seedlings. Illumination of seedlings also promoted an earlier, and more rapid decline of isocitrate lyase transcripts. Peak levels of isocitrate lyase mRNA preceded a corresponding peak in immunologically detectable isocitrate lyase polypeptides by about 24 hours. Isocitrate lyase expression in sunflower cotyledons is developmentally regulated and is modulated, in seedlings, by exposure to light. Mechanisms that control these processes appear to function primarily at the level of mRNA accumulation and are likely to involve changes in transcription rates and/or mRNA stability.

Cottonseed malate synthase : biogenesis in maturing and germinated seeds

The activity of malate synthase (MS) (EC 4.1.3.2) appears and increases during cotton (Gossypium hirsutum L.) seed maturation, persists through desiccation and imbibition, then increases again following germination. The research reported herein is a comparative study of the synthesis and acquisition of MS into glyoxysomes as they occur in maturing and germinated seeds. Rate-zonal centrifugation of cotyledon extracts revealed that the 5 Svedberg unit (S) cytosolic form of MS was the only form present at 42 days postanthesis (DPA) when activity was first detectable. At later stages (48 DPA, 0 day, 26 hours, and 48 hours), both the 5S and glyoxysomal 20S forms were present, with the 20S form becoming much more prevalent. Western blot analyses revealed that no other form(s) of MS were present in the phosphate-buffered gradients, and that 5S and 20S forms had the same subunit molecular weight in maturing and germinated seeds. Comparisons of radiospecific activity of MS immunoprecipitates following in vivo labeling with [(35)S]methionine for varying time intervals provided strong evidence for a 5S-precursor to 20S-product relationship during both seed maturation and seedling growth. Comparisons of MS labeled in vivo and in vitro in wheat germ and rabbit reticulocyte lysates programmed with poly(A)(+)RNA (from maturing and germinated seeds) revealed no detectable differences in subunit molecular weights. These results reinforced our other data indicating that MS was synthesized in the cytosol and acquired by glyoxysomes in both maturing and germinated cotton seeds without involvement of an intervening aggregate pool in the endoplasmic reticulum, or via processing of a cleavable precursor molecule. MS was translated from poly(A)(+)RNA extracted from 28 DPA cotton seeds. This was nearly 2 weeks before MS activity or protein was detected in vivo. This finding invites further study on the regulation of RNA transcripts during maturation.

Cottonseed malate synthase : purification and immunochemical characterization

Malate synthase (EC 4.1.3.2), an enzyme unique to the glyoxylate cycle, was purified to homogeneity from cotyledons of 72-hours, darkgrown cotton (Gossypium hirsutum L.) seedlings. Homogeneity of the enzyme was assessed by silver staining SDS-PAGE gels. Purification was accomplished by using a single buffer medium through six steps involving one ammonium sulfate fractionation and chromatography on three columns (Sephacryl S-300, DEAE Sephacel, Phenyl Sepharose). Large-scale preparation of glyoxysomes, a main step in all other published procedures, was not involved. The purified enzyme and that extracted from glyoxysomes appears to be a dodecamer with a native molecular weight of 750,000 (sedimentation coefficient of >20 Svedberg units [S] on sucrose gradients) composed of identical subunits (molecular weight approximately 63,000). The monomer (5S) occurs in the cytosol. Polyclonal antibodies raised in rabbits were judged to be monospecific for malate synthase by immunotitration, double immunodiffusion, and western blotting. Double immunodiffusion experiments revealed only partial immunological identity between the 5S (cytosolic) and 20S (glyoxysomal forms, although complete identity was observed between the 5S form in immature and germinated seeds, and the 20S form in immature and germinated seeds. Cross-reactivity of the cotton antimalate synthase serum was observed with extracts from five other oilseeds. Western blot analyses showed that malate synthase protein was not present in immature seeds prior to appearance of enzyme activity, but when present, subunit molecular weight was indistinguishable in immature, desiccated, and germinated seeds.

Heterogeneity of catalase in maturing and germinated cotton seeds

To investigate possible charge and size heterogeneity of catalase (EC 1.11.1.6) in cotton (Gossypium hirsutum L. cv Deltapine 62), extracts of cotyledons from different developmental ages were subjected to nondenaturing polyacrylamide gel electrophoresis and isoelectric focusing. Special precautions (e.g. fresh homogenates, reducing media) were necessary to prevent artefacts due to enzyme modification during extraction and storage. When the gels were stained for enzyme activity, two distinct electrophoretic forms of catalase were resolved in extracts of maturing and mature cotton seeds. In germinated seeds, three additional cathodic forms were detected revealing a total of five electrophoretic variants. In green cotyledons, the two anodic forms characteristic of ungerminated seeds were less active; whereas, the most cathodic form was predominant. All forms of catalase were found in isolated glyoxysomes. Corresponding electrophoretic patterns were found on Western blots probed with anticatalase serum; no immunoreactive, catalytically inactive forms were detected. Western blots of sodium dodecyl sulfate-polyacrylamide gels revealed only one immunoreactive (55 kilodaltons) polypeptide in cotton extracts of all developmental ages. Results from isoelectric focusing and Ferguson plots indicate that the electrophoretic variants of catalase are charge isomers with a molecular weight of approximately 230,000.

Ontogeny of glyoxysomes in maturing and germinated cotton seeds-a morphometric analysis

Morphometric procedures were used with light and electron microscopy to examine glyoxysome number, volume, shape and distribution as well as mesophyll cell volume, in cotyledons of mature (50 d postanthesis), imbibed (5h) and germinated (24 and 37 h) cotton (Gossypium hirsutum L.) seeds. Additionally, activities of five glyoxysomal marker enzymes in cotyledon extracts were assayed at each of the above ages. Cell volume was determined from photomicrographs of Epon-embedded sections by the point-counting procedure. Analysis of variance showed that cell volume was not different among the tissue segments studied. Glyoxysomes were cytochemically stained for catalase (EC 1.11.1.6) activity with the 3,3'-diaminobenzidine-tetrahydrochloride procedure. Analyses involving both phase and electron microscopy, and two separate sterologic calculations for determining the number of glyoxysomes per cell, indicate that glyoxysomes are numerous in mature seeds, persist through desiccation and imbibition, then increase dramatically in volume (seven fold) but not number (a maximum of 1.5-fold), when enzyme activities increase two to six times (depending on the enzyme). During the entire period of increase in glyoxysomal enzyme activities, no ultrastructural evidence was found for glyoxysome formation or destruction. Our data, in contrast to some proposals in the literature, indicate that cottonseed glyoxysomes form during seed maturation, then develop following seed imbibition into pleomorphic organelles by posttranslational accumulation of proteins from the cytosol and transfer of membrane components probably from the endoplasmic reticulum.

Metabolism of carbohydrate and lipid reserves in germinated cotton seeds

Utilization of reserve lipid and carbohydrates during germination (0-12 h) and postgerminative growth (12-48 h) was studied in cotton (Gossypium hirsutum L.) seedlings. Raffinose and stachyose were utilized during the germination period and early growth; mobilization was associated with α-galactosidase (EC 3.2.1.22) activity. Results from pulse-chase experiments with [(3)H]raffinose supplied exogenously to 4-h soaked seeds indicated that raffinose-derived catabolites contributed to the coincident increase in cotyledon sucrose and starch, and to the small increase in axis dry weight. Starch appears to be an alternative sink for end products of hydrolysis of reserve carbohydrates prior to the onset of rapid axis growth and cotyledon expansion. Mobilization of neutral lipid commenced at about 16 h after soaking, concomitant with development of key glyoxylate-cycle and other gluconeogenesis-related enzyme activities. Axis dry weight increased three-fold between 24 and 48 h. Results from pulse-chase (3 h, 16 h) experiments in which [2-(14)C]acetate was supplied to cotyledons of intact 22-h-old seedlings showed that acetate-derived metabolites were not transported exclusively to the axes, but were partitioned between axes and cotyledons. Only 27% of total incorporated radioactivity was recovered in axes following the chase, 18% was evolved as CO2, and the rest was recovered in water-soluble substances (20%) and polymers (31%) within the cotyledons. Of the polymers, 55% of the activity was in polysaccharides (Starch, pectic substances, hemicellulose, cellulose), 25% in protein, and 20% in unidentified neutral and acidic compounds. Considering these data, the amount of lipid mobilized, and various routes by which supplied [2-(14)C]acetate could be metabolized, it appears that lipidderived compounds contribute only 25-40% of axis dry-weight gain. Lipid-derived substances retained in the cotyledons likely are utilized for expansion and differentiation of the cotyledons into photosynthetic organs.

Control of enzyme activities in cotton cotyledons during maturation and germination : III. In-vitro embryo development in the presence of abscisic acid

Cotton (Gossypium hirsutum L.) embryos excised from bolls 38-43 d after anthesis and cultured in vitro for 4 d on a nutrient agar medium containing 3.8 μM abscisic acid (ABA) developed enzyme activity and accumulated insoluble protein, neutral lipid, and dry weight similar to embryos maturing on the plant. Inclusion of ABA in the medium prevented precosious germination and allowed continued increases in catalase, malate dehydrogenase, citrate synthase, aspartate aminotransferase, and β-oxidation enzyme activities as well as de-novo synthesis of malate synthase. Isocitrate lyase activity was not detectable in ABA-cultured embryos nor normally-developed embryos. Omission of sucrose from the medium resulted in near-doubling of the development of malate synthase activity, with minimal effects on the other enzyme activities. Addition of Actinomycin D, cordycepin, or cycloheximide to ABA-containing cultures did not overcome the observed inhibition of germination, but severely reduced both the appearance of new malate synthase activity and further production of other related enzyme activities. Thus, development of these enzyme activities in the presence of ABA appears dependent on transcription and translation, while inhibition of germination by ABA at this stage of development is not sensitive to the RNA- and protein-synthesis inhibitors. The results indicate that ABA does not prevent vivipary by suppressing translation of m-RNAs coding for isocitrate lyase and its companion enzymes, as previously proposed.

Enzyme development and glyoxysome characterization in cotyledons of cotton seeds

Unimbibed, mature cotton seeds (cv. Deltapine 61) were found to possess activity for all gluconeogenesis-related enzymes examined, except for isocitrate lyase activity. This indicates that transcription and translation of most enzymes needed for postgerminative growth takes place during seed maturation. This is in contrast with the generalization that "germination" enzymes are synthesized de novo from previously untranslated mRNAs conserved in dry seeds. All enzyme activities increased 3-fold or greater following imbibition, and most remained constant after reaching their peak. Notable exceptions were activities for three beta oxidation enzymes and fructose bisphosphatase, which decreased precipitously after peaking with other enzyme activities.Standard sucrose gradient procedures with swing-out rotors were not useful for isolating cotton glyoxysomes. Satisfactory and reproducible results ultimately were obtained with sucrose gradients constructed in a Beckman JCF-Z zonal rotor. Specific activities of glyoxysomal enzymes were 2- to 3-fold lower than those reported for other oil seeds, except malate dehydrogenase which was 10-fold lower. Electron microscopy revealed that protein body fragments were the primary contaminant of glyoxysome fractions. Glyoxysomes were subfractionated by osmotic shock treatments to evaluate sub-organelle localization of constituent enzymes, several of which have not been examined in other oil seeds.

Role of malate synthase in citric Acid synthesis by maturing cotton embryos: a proposal

Cotton embryos from 34 to 54 days after anthesis were analyzed for organic acids, and enzymes associated with organic acid metabolism. During this developmental period, embryos accumulated citrate. Malate synthase activity appeared at 46 days after anthesis and increased rapidly to 54 days. Of other enzymes examined, only citrate synthase activity increased during this period. As isocitrate lyase activity was absent from cotton embryos during maturation, an alternative source of glyoxylate would be required for in vivo malate synthase activity. Of several metabolic sources tested, glycine was converted to glyoxylate via a transamination reaction.Halves of 50-day (mature) cotton embryos incorporated radioactivity from [1-(14)C]acetate, [1-(14)C]glyoxylate, and [1-(14)C]glycine into organic acids. Embryo halves incubated with [(14)C]glyoxylate plus [(3)H]acetate synthesized double-labeled malate and citrate. Radioactive citrate isolated from 50-day cotton embryos incubated with [1-(14)C]acetate was degraded; label was distributed as follows: 55% in C(1), 33% in C(5), and 12% in C(6). Taken together, these data strongly suggest participation of malate synthase in citrate production in vivo.Separation of organelles by sucrose density gradient sedimentation revealed that malate synthase, malate dehydrogenase, and citrate synthase were compartmentalized together only in the peroxisome fraction (1.24 grams per milliliter). Peroxisomes isolated from 50-day embryos, when incubated with glyoxylate and [(3)H]acetyl-CoA, synthesized labeled malate and citrate, but only radioactive citrate accumulated. Incubations with glycine plus alpha-ketoglutarate, in place of glyoxylate, also resulted in synthesis of radioactive citrate.A metabolic scheme illustrating the participation of cotton embryo peroxisomes in citrate synthesis is proposed. This scheme suggests a function for plant peroxisomes not previously elucidated. The ontogenetic and metabolic relationship between these organelles and glyoxysomes active in gluconeogenesis during postgerminative growth remains to be examined.

Control of Enzyme Activities in Cotton Cotyledons during Maturation and Germination : IV. beta-OXIDATION

Microbodies were isolated by zonal-rotor sucrose density gradient centrifugation from cotton (cv. DP 61) seeds at two distinct stages of embryogenesis (38 and 50 days after anthesis) and after 48 hours postgerminative growth. In all cases, beta-oxidation activity (palmitoyl-coenzyme A (CoA)-dependent reduction of acetylpyridine adenine dinucleotide or production of acetyl-CoA) and activities of the enzymes palmitate:CoA ligase, acyl-CoA oxidase, enoyl hydratase, 3-hydroxyacyl-CoA dehydrogenase, and 3-oxoacyl-CoA thiolase, plus catalase, were localized exclusively in the microbody fractions, i.e. none of the activities were associated with mitochondria. Acyl-CoA dehydrogenase activity could not be detected in any of the gradient fractions or in homogenates.Glyoxysomes isolated from cotyledons of 48-hour-germinated seeds were capable of beta-oxidation of acyl-CoAs of various chain lengths and degrees of unsaturation and were the sole site of 3-cis-2-trans enoyl-CoA isomerase activity. Direct measurement of the isomerase is the first demonstration of an enzyme required for unsaturated fatty acid catabolism in a higher plant. Palmitoyl-carnitine was not oxidized by any of the organelle fractions.Subfractionation of glyoxysomes by osmotic shock revealed that none of the beta-oxidation enzymes were tightly membrane-associated.

Malate synthase activity in cotton and other ungerminated oilseeds: a survey

Extracts from several species and varieties of ungerminated cotton seeds plus homogenates from 18 other oilseeds (representing 11 different families) were examined for malate synthase and isocitrate lyase activity. Malate synthase activities in the various cotton seeds ranged from 35 to 129% of the units per dry seed weight found in Deltapine 16 cotton. For other oilseeds, the range was from 0.3 to 58% of Deltapine 16 cotton. Castor bean (Ricinus communis L.) had the least activity per mg dry weight (12-fold lower than the next lowest species), while Pima cotton (Gossypium barbadense L.) had the highest level (8.53 units). On a per seed basis, these values were 15 and 747 nanomoles per minute.Malate synthase activity was measurable in all seed types examined, whereas isocitrate lyase could not be detected in any of the seeds. We suggest that synthesis of malate synthase during seed development is universal among oilseeds in the absence of glyoxylate-cycle-associated isocitrate lyase activity.

Control of Enzyme Activities in Cotton Cotyledons during Maturation and Germination: II. Glyoxysomal Enzyme Development in Embryos

The sequence of glyoxysomal enzyme development was investigated in cotyledons of cotton (Gossypium hirsutum L. cv. Deltapine 16) embryos from 16 to 70 days after anthesis (DAA). Catalase, malate dehydrogenase, and citrate condensing enzyme activities were barely detectable prior to 22 DAA, but showed dramatic increases from 22 to 50 DAA. Development of malate synthase activity, however, was delayed during this period, rising to peak activity from 45 to 50 DAA (just prior to desiccation) in the absence of any detectable isocitrate lyase activity. Substantial activities of all of these enzymes (except isocitrate lyase) persisted in the dry seeds. Isopycnic centrifugations on sucrose gradients demonstrated that the enzymes were compartmentalized within particles increasing in buoyant density with time of development (1.226 to 1.245 grams per cubic centimeter from 22 to 50 DAA). Of particular significance were the observations in 22-day embryos of smooth surfaced membrane dilations of rough endoplasmic reticulum having cytochemical catalase reactivity, and the demonstrations of catalase activities in microsomal fractions isolated throughout the 16- to 50-DAA period. Our data do not allow determination of the mechanism(s) for enzyme activation and/or addition to previously existing or newly formed microbodies, but do show that development and acquisition of enzyme activities within glyoxysomes occur sequentially and thus are not regulated in concert as previously thought.

Subcellular localization of glyoxylate cycle enzymes in Ascaris suum larvae

Evidence is presented on the particulate nature of glyoxylate cycle enzymes in metazoa with the use of 15-day old larvae of the nematode Ascaris suum. Homogenization procedures were developed to disrupt the resistant nematode cuticle. Malate synthase and isocitrate lyase, key enzymes of the glyoxylate cycle, consistently sedimented with mitochondrial enzymes in differential pellets while catalase, a major peroxisomal enzyme, was always soluble. Isopycnic sucrose gradient centrifugation of the differential pellet yielded two protein peaks: one at 1.18 g/cm3 (characteristic for mitochondria), and another at 1.23 g/cm3 (common for glyoxysomes and peroxisomes). Electron microscopy of these fractions revealed that the lighter peak consisted primarily of mitochondria, while the heavier band contained proteinaceous bodies termed "dense granules" morphologically resembling microbodies. SIgnificantly, both malate synthase and isocitrate lyase cosedimented with the mitochondrial marker enzymes in the lighter peak (1.18 g/cm3) and not with the dense granules. Further purification of mitochondria, accomplished by separating dense granules with a step gradient before isopycnic centrifugation, substantiated the evidence that microbodies (glyoxysomes) do not occur in these nematode larvae. Rough-surfaced membranes were alternatively considered as the subcellular site, but the evidence tends to favor localization of the glyoxylate bypass enzymes in the mitochondria.

Cytochemical demonstration of malate synthase and glycolate oxidase in microbodies of cucumber cotyledons

The cytochemical localizations of malate synthase (glyoxysomal marker) and glycolate oxidase (peroxisomal marker) have been examined in cotyledon segments and sucrose-gradient fractions from germinated cucumber (Cucumis sativus L.) seedlings. The seedlings were grown in the dark for 4 days, transferred to 4 hours of continuous light, then returned to the dark for 24 hours. Under these conditions, high specific activities for both glyoxysomal and peroxisomal enzymes are maintained in cotyledon homogenates and microbody-enriched fractions. Electron cytochemistry of the marker enzymes reveals that all or virtually all the microbodies observed in cotyledonary cells and sucrose-gradient fractions contain both enzymes. The staining in gradient fractions was determined from scoring a minimum of 600 photographed microbodies for each enzyme. After correcting for the number of particles stained for catalase reactivity (representing true microbodies), 94 and 97% of the microbodies were found stained for malate synthase and glycolate oxidase activity, respectively.The results from these studies provide pertinent information toward understanding the succession from glyoxysomal to peroxisomal metabolism in cotyledons of fatty seedlings. The coexistence of two separate microbody types functioning at different stages of development apparently is not the case. The localizations of both marker enzymes within one microbody type strongly suggest that the metabolic transition involves a change in enzyme complement within an ongoing population of microbodies.

Ultrastructure of developing Ascaris larvae undergoing lipid to carbohydrate interconversion

Triglyceride utilization is correlated with glycogen resynthesis in developing Ascaris suum larvae. Glycogen content, determined by amyloglucosidase hydrolysis at 2-day intervals, decreases sharply during embryonation from 15% (per mg dry weight) at day 4 to 2% at day 12. Thereafter glycogen increases 3-fold through day 20, marking the resynthesis period. Triglyceride lipid droplets are confirmed primarily to the posterior half of the larvae and mark the site of the interconversion. Although they serve as precursors to glycogen, only a small diminution was observed ultrastructurally. Glycogen accumulation, on the other hand, correlates well with increases determined biochemically. alpha-glycogen builds up among lipid droplets, while beta-glycogen concentrates in the cytoplasm of somatic muscle cells paralleling myofibril development. Dense granules, restricted to the lipid body region, are considered as the possible subcellular site for the enzymatic conversion.