Immunocytochemical localization of phaseolin and phytohemagglutinin in the endoplasmic reticulum and Golgi complex of developing bean cotyledons

The proteolytic processing of seed storage proteins in Arabidopsis embryo cells starts in the multivesicular bodies

We have investigated the transport of storage proteins, their processing proteases, and the Vacuolar Sorting Receptor-1/Epidermal Growth Factor Receptor-Like Protein1 (VSR-1/ATELP1) receptor during the formation of protein storage vacuoles in Arabidopsis thaliana embryos by means of high-pressure freezing/freeze substitution, electron tomography, immunolabeling techniques, and subcellular fractionation. The storage proteins and their processing proteases are segregated from each other within the Golgi cisternae and packaged into separate vesicles. The storage protein-containing vesicles but not the processing enzyme-containing vesicles carry the VSR-1/ATELP1 receptor. Both types of secretory vesicles appear to fuse into a type of prevacuolar multivesicular body (MVB). We have also determined that the proteolytic processing of the 2S albumins starts in the MVBs. We hypothesize that the compartmentalized processing of storage proteins in the MVBs may allow for the sequential activation of processing proteases as the MVB lumen gradually acidifies.

AtRMR1 functions as a cargo receptor for protein trafficking to the protein storage vacuole

Organellar proteins are sorted by cargo receptors on the way to their final destination. However, receptors for proteins that are destined for the protein storage vacuole (PSV) are largely unknown. In this study, we investigated the biological role that Arabidopsis thaliana receptor homology region transmembrane domain ring H2 motif protein (AtRMR) 1 plays in protein trafficking to the PSV. AtRMR1 mainly colocalized to the prevacuolar compartment of the PSV, but a minor portion also localized to the Golgi complex. The coexpression of AtRMR1 mutants that were localized to the Golgi complex strongly inhibited the trafficking of phaseolin to the PSV and caused accumulation of phaseolin in the Golgi complex or its secretion. Coimmunoprecipitation and in vitro binding assays revealed that the lumenal domain of AtRMR1 interacts with the COOH-terminal sorting signal of phaseolin at acidic pH. Furthermore, phaseolin colocalized with AtRMR1 on its way to the PSV. Based on these results, we propose that AtRMR1 functions as the sorting receptor of phaseolin for its trafficking to the PSV.

Vacuolar storage proteins are sorted in the cis-cisternae of the pea cotyledon Golgi apparatus

Developing pea cotyledons contain functionally different vacuoles, a protein storage vacuole and a lytic vacuole. Lumenal as well as membrane proteins of the protein storage vacuole exit the Golgi apparatus in dense vesicles rather than in clathrin-coated vesicles (CCVs). Although the sorting receptor for vacuolar hydrolases BP-80 is present in CCVs, it is not detectable in dense vesicles. To localize these different vacuolar sorting events in the Golgi, we have compared the distribution of vacuolar storage proteins and of alpha-TIP, a membrane protein of the protein storage vacuole, with the distribution of the vacuolar sorting receptor BP-80 across the Golgi stack. Analysis of immunogold labeling from cryosections and from high pressure frozen samples has revealed a steep gradient in the distribution of the storage proteins within the Golgi stack. Intense labeling for storage proteins was registered for the cis-cisternae, contrasting with very low labeling for these antigens in the trans-cisternae. The distribution of BP-80 was the reverse, showing a peak in the trans-Golgi network with very low labeling of the cis-cisternae. These results indicate a spatial separation of different vacuolar sorting events in the Golgi apparatus of developing pea cotyledons.

Mass transport of proform of a KDEL-tailed cysteine proteinase (SH-EP) to protein storage vacuoles by endoplasmic reticulum-derived vesicle is involved in protein mobilization in germinating seeds

A vacuolar cysteine proteinase, designated SH-EP, is expressed in the cotyledon of germinated Vigna mungo seeds and is responsible for the degradation of storage proteins. SH-EP is a characteristic vacuolar proteinase possessing a COOH-terminal endoplasmic reticulum (ER) retention sequence, KDEL. In this work, immunocytochemical analysis of the cotyledon cells of germinated V. mungo seeds was performed using seven kinds of antibodies to identify the intracellular transport pathway of SH-EP from ER to protein storage vacuoles. A proform of SH-EP synthesized in ER accumulated at the edge or middle region of ER where the transport vesicle was formed. The vesicle containing a large amount of proSH-EP, termed KV, budded off from ER, bypassed the Golgi complex, and was sorted to protein storage vacuoles. This massive transport of SH-EP via KV was thought to mediate dynamic protein mobilization in the cotyledon cells of germinated seeds. We discuss the possibilities that the KDEL sequence of KDEL-tailed vacuolar cysteine proteinases function as an accumulation signal at ER, and that the mass transport of the proteinases by ER-derived KV-like vesicle is involved in the protein mobilization of plants.

Transport of storage proteins to the vacuole is mediated by vesicles without a clathrin coat

Storage parenchyma cells of developing legume cotyledons actively transport large amounts of storage proteins to protein storage vacuoles (PSV). These proteins are synthesized on the endoplasmic reticulum and pass through the Golgi apparatus. Clathrin coated vesicles (CCV) and small electron dense vesicles found near the trans-Golgi network (TGN) have both been implicated in the Golgi-to-vacuole transport step. Recent findings that protein storage cells contain more than one type of vacuole have necessitated a re-examination of the role of both types of vesicles in vacuolar protein transport. Immunoblots of highly purified CCV preparations and immunogold labelling with antibodies to the storage proteins vicilin and legumin, indicate that the dense vesicles, but not the CCV, are involved in storage protein transport in pea cotyledons. This result is supported by the finding that alpha-TIP, a protein characteristic of the PSV membrane, is absent from CCV. In addition, complex glycoproteins appear to be carried by CCV but are not detectable in the PSV. We suggest on the basis of these data that storage proteins and other vacuolar proteins such as acid hydrolases are not sorted by the same mechanism and are transported by different types of vesicles to different types of vacuoles.

Protein sorting in the endomembrane system of plant cells

Although many properties of the targeting of plant endomembrane proteins are similar to mammalian and yeast systems, several clear differences are found that will be stressed in this review. In the past year, significant advances in our understanding of storage protein segregation in the endoplasmic reticulum, compartmentation of Golgi, and the signals for vacuolar protein targeting have been made. This work will form the basis for determining the mechanism of these sorting phenomena.

Redistribution of a Golgi glycoprotein in plant cells treated with Brefeldin A

The fungal fatty acid derivative Brefeldin A (BFA), has been used to study the reversible distribution of a Golgi glycoprotein, the JIM 84 epitope, into the cytosol of higher plant cells. Treatment of both maize and onion root tip cells resulted in a rearrangement of the Golgi stacks into either circular formations or a perinuclear distribution. The Golgi cisternae became curved and vesiculated and in cells where the Golgi apparatus was totally dispersed the JIM 84 epitope was associated with large areas in the cytosol which were also vesiculated. On removal of the BFA the Golgi apparatus reformed and the JIM 84 epitope was again located in the cisternal stacks. This mode of BFA action is compared with that so far described for animal cells.

The S-locus specific glycoproteins of Brassica accumulate in the cell wall of developing stigma papillae

Self-incompatibility in Brassica oleracea is now viewed as a cellular interaction between pollen and the papillar cells of the stigma surface. In this species, the inhibition of self-pollen occurs at the stigma surface under the influence of S-locus specific glycoproteins (SLSG). We used antibodies specific for a protein epitope of SLSG to study the subcellular distribution of these molecules in the stigmatic papillae. The antibodies have uncovered an interesting epitope polymorphism in SLSG encoded by subsets of S-alleles, thus providing us with useful genetic controls to directly verify the specificity of the immunolocalization data. Examination of thin sections of Brassica stigmas following indirect immunogold labeling showed that SLSG accumulate in the papillar cell wall, at the site where inhibition of self-pollen tube development has been shown to occur. In addition, the absence of gold particles over the papillar cell walls in the immature stigmas of very young buds, and the intense labeling of these walls in the stigmas of mature buds and open flowers, correlates well with the acquisition of the self-incompatibility response by the developing stigma.

Immunolocalization of avenin and globulin storage proteins in developing endosperm of Avena sativa L

The seed storage proteins of oats (Avena sativa L.) are synthesized and assembled into vacuolar protein bodies in developing endosperm tissue. We used double-label immunolocalization to study the distribution of these proteins within protein bodies of the starchy endosperm. When sections of developing oat endosperm sampled 8 d after anthesis were stained with uranyl acetate and lead citrate, the vacuolar protein bodies consisted of light-staining regions which were usually surrounded by a darker-staining matrix. Immunogold staining of this tissue demonstrated a distinct segregation of proteins within protein bodies; globulins were localized in the dark-staining regions and prolamines were localized in the light-staining regions. We observed two additional components of vacuolar protein bodies: a membranous component which was often appressed to the outside of the globulin, and a granular, dark-staining region which resembled tightly clustered ribosomes. Neither antibody immunostained the membranous component, but the granular region was lightly labelled with the anti-globulin antibody. Anti-globulin immunostaining was also observed adjacent to cell walls and appeared to be associated with plasmodesmata. Immunostaining for both antigens was also observed within the rough endoplasmic reticulum. Based on the immunostaining patterns, the prolamine proteins appeared to aggregate within the rough endoplasmic reticulum while most of the globulin appeared to aggregate in the vacuole.

Formation of wheat protein bodies: Involvement of the Golgi apparatus in gliadin transport

Developing wheat (Triticum aestivum L.) endosperm was examined using ultrathin sections prepared from tissues harvested at 5, 9, 16 and 25 d after flowering. Protein bodies were evident by 9 d and displayed a variety of membranous structures and inclusions. The Golgi apparatus was a prominent organelle at all stages, and by 9 d was associated with small electron-dense inclusions. By immunocytochemical techniques, gliadin (wheat prolamine) was localized within these vesicles and in homogeneous regions of protein bodies, but not in the lumen of the rough endoplasmic reticulum. The protein bodies appear to enlarge by fusion of smaller protein bodies resulting in larger, irregular-shaped organelles. The affinity of the Golgi-derived vesicles for gliadin-specific probes during the period of maximal storage-protein synthesis and deposition indicates that this organelle includes the bulk, if not all, of the gliadin produced. The involvement of the Golgi apparatus in the packaging of gliadins into protein bodies indicates a pathway which differs from the mode of prolamine deposition in other cereals such as maize, rice and sorghum, and resembles the mechanism employed for the storage of rice glutelin and legume globulins.

Immunogold-localization and synthesis of an oil-body membrane protein in developing soybean seeds

The synthesis of a major oil-body membrane brotein was studied in maturing soybean (Glycine max (L.) Merr.) cotyledons. The membrane contained four abundant proteins with apparent molecular mass (Mr) of 34000, 24000, 18000 and 17000. The Mr=24000 protein (mP 24) was selected for more detailed analysis. The protein was purified to apparent homogeneity by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and isolated from the gel by electroelution or chemical hydrolysis of gel crosslinks. It was then used to elicit rabbit antibodies which were judged to be specific when assayed by SDS-PAGE-immunoblot procedures. The mP 24 was localized in immature soybean cotyledon cells by indirect immunogold procedures on thin sections of Lowicryl- and LR-White-embedded tissue. Indirect labeling with the primary antiserum followed by colloidal gold-protein A showed specific labeling of the oil-body membrane and an absence of label on the other subcellular organelles including the endoplasmic reticulum (ER). Parallel tissue samples were studied by conventional transmission electron microscopy. Although segments of the ER were observed to be closely juxtaposed to the oil bodies, continuity between the two organelles was not observed. The synthesis of mP 24 was studied by in-vitro translation and in-vivo labeling with [(3)H]leucine followed by indirect immunoaffinity isolation of the labeled products. The SDS-PAGE fluorography results indicated that the primary translation product and the in-vivo synthesized protein have the same Mr, and this is also the same Mr as the protein in the mature membrane.

Immunochemical studies on the role of the Golgi complex in protein-body formation in rice seeds

Antibodies raised against purified glutelins and prolamines were employed as probes to study the cellular routes by which these proteins are deposited into protein bodies of rice (Oryza sativa L.) endosperm. Three morphologically distinct protein bodies, large spherical, small spherical, and irregularly-shaped, were observed, in agreement with existing reports. Immunocytochemical studies showed the presence of glutelins in the irregularly-shaped protein bodies while the prolamines were found in both the large and small spherical protein bodies. Both the large and small spherical protein bodies, distinguishable by electron density and gold-labeling patterns, appear to be formed by direct deposition of the newly formed proteins into the lumen of the rough endoplasmic reticulum (ER). In contrast, glutelin protein bodies are formed via the Golgi apparatus. Small electron-lucent vesicles are often found at one side of the Golgi. Electron-dense vesicles, whose contents are labeled by glutelin antibody-gold particles, are commonly observed at the distal side of the Golgi apparatus and fuse to form the irregularly shaped protein bodies in endosperm cells. These observations indicate that the transport of rice glutelins from their site of synthesis, the ER, to the site of deposition, the protein bodies, is mediated by the Golgi apparatus.

Immunoelectron-microscopy localization of abscisic acid with colloidal gold on Lowicryl-embedded tissues of Chnopodium polyspermum L

Further study on the localization of abscisic acid (ABA) has been undertaken at the ultrastructural level in Chenopodium polyspermum L. Axillary-bud-bearing nodes on the main axis were fixed with soluble 1-(3-dimethylaminopropyl)-3 ethyl carbodiimide, then postfixed with paraformaldehyde and embedded in Lowicryl K4M at-20° C. Ultrathin sections mounted on grids were successively incubated with rabbit anti-ABA antibodies and with gold-labelled goat anti-rabbit anti-bodies (40 nm particle size). Control sections treated with preimmune rabbit serum and ABA-preabsorbed antibodies were devoid of label. The background staining was very low with this technique. Quantitative analysis of the immunolabelling showed that two main sites of ABA accumulation could be defined: first, plastids in cortical cells and vascular parenchyma cells associated with sieve elements and xylem vessels; second, the cell cytoplasm and nucleus in the axillary bud tip and in procambial strands. In vascular bundles, the cambial cells showed no immunoreactivity. These observations support the hypothesis for the cytoplasmic synthesis of ABA which is subsequently trapped in plastids as cells mature.

Involvement of the Golgi apparatus in the secretion of α-amylase from gibberellin-treated barley aleurone cells

Localisation of α-amylase (EC 3.2.1.1) in barley aleurone cells treated with gibberellic acid has been achieved using protein A-gold-labelled polyclonal antibodies. Gold particles were located almost exclusively over the lumen of the rough endoplasmic reticulum and cisternae of the Golgi apparatus. The label was most concentrated over the Golgi apparatus. This indicates that the Golgi is involved in the secretion of α-amylase protein from aleurone cells.