Biosynthesis and processing of legumin-like storage proteins in Lupinus angustifolius (lupin)

A not-glycosylated isoform of γ-conglutin, a hexameric glycoprotein of Lupinus albus seed, participates in the oligomerization equilibrium

γ-conglutin (γ-C) is a hexameric glycoprotein accumulated in lupin seeds and has long been considered as a storage protein. Recently, it has been investigated for its possible postprandial glycaemic regulating action in human nutrition and for its physiological role in plant defence. The quaternary structure of γ-C results from the assembly of six monomers in reversible pH-dependent association/dissociation equilibrium. Our working hypothesis was that the γ-C hexamer is made up of glycosylated subunits in association with not-glycosylated isoforms, that seem to have 'escaped' the correct glycosylation process in the Golgi. Here we describe the isolation of not-glycosylated γ-C monomers in native condition by two in tandem lectin-based affinity chromatography and the characterization of their oligomerization capacity. We report, for the first time, the observation that a plant multimeric protein may be formed by identical polypeptide chains that have undergone different post-translational modifications. All obtained considered, the results strongly suggest that the not-glycosylated isoform can also take part in the oligomerization equilibrium of the protein.

Protein distribution in lupin protein isolates from Lupinus angustifolius L. prepared by various isolation techniques

Differences in the protein distribution of various protein isolates from Lupinus angustifolius L. Vitabor were identified as affected by the isolation procedure (alkaline and/or salt-induced extraction followed by isoelectric and/or dilutive precipitation). Protein isolates extracted in alkaline solution showed higher protein yields (26.4-31.7%) compared to salt-induced extraction (19.8-30.0%) or combined alkaline and salt-induced extraction (23.3-25.6%). Chemical variations among the protein isolates especially occurred within the albumins. Protein isolates precipitated isoelectrically showed the highest contents, whereas protein isolates precipitated by dilutive showed the lowest contents of conglutin δ. Furthermore, the alkaline subunits of conglutin α and conglutin γ decreased during alkaline extraction compared to salt-induced extraction. A decrease in protein-bound polar and basic amino acids was shown after protein isolation. In contrast, the amounts of nonpolar, aliphatic, aromatic, hydroxylated and sulfur-rich amino acids were higher in the lupin protein isolates compared to the lupin flakes. However, the functional side chains could not be related to the specific molecular arrangements of the protein isolates, as a similar amino acid composition was found among the protein isolates.

Transcription of genes for conglutin gamma and a leginsulin-like protein in narrow-leafed lupin

The expression of genes encoding conglutin gamma and a leginsulin-like protein has been examined in narrow-leafed lupin, Lupinus angustifolius L. Conglutin gamma is a homologue of basic 7S globulin (Bg), the insulin and leginsulin binding protein from soybean. Accumulation of conglutin gamma mRNA, as assessed by northern assays and reverse-transcription PCR, was tightly regulated both spatially and temporally in lupin plants and was detected almost exclusively in developing seeds. Similar tissue and temporal specificity was demonstrated when 1.8 kb of the promoter region from the conglutin gamma gene was used to drive the expression of a beta-glucuronidase reporter gene in transgenic plants. In stably transformed tobacco the conglutin gamma promoter produced strong, temporally regulated and seed-specific expression of the reporter gene which was localised to the embryo tissues and to a layer of cells adjacent to the seed coat. A truncated 0.29 kb promoter fragment produced much reduced levels of expression and a loss of embryo specificity. Leginsulin-like mRNA was similarly detected in lupins only in developing seeds. The leginsulin-like gene detected in L. angustifolius showed 96% sequence identity to leginsulin from soybean within the 280 bp region amplified from lupin by PCR. The results demonstrate that both components of a Bg-leginsulin putative signal transduction pathway are present in the seeds of lupin.

Degradation of transport-competent destabilized phaseolin with a signal for retention in the endoplasmic reticulum occurs in the vacuole

To understand how plant cells exert quality control over the proteins that pass through the secretory system we examined the transport and accumulation of the bean (Phaseolus vulgaris L.) vacuolar storage protein phaseolin, structurally modified to contain a helix-breaking epitope and carboxyterminal HDEL, an endoplasmic reticulum (ER)-retention signal. The constructs were expressed in tobacco (Nicotiana tabacum L.) with a seed-specific promoter. The results show that phaseolin-HDEL accumulates in the protein-storage vacuoles, indicating that HEDL does not contain sufficient information for retention in the ER. However, the ER of seeds expressing the phaseolin-HDEL construct contain relatively more phaseolin-HDEL compared to phaseolin in the ER of seeds expressing the phaseolin construct. This result indicates that the flow out of the ER is retarded but not arrested by the presence of HDEL. Introduction into phaseolin of the epitope "himet" (Hoffman et al., 1988, Plant Mol. Biol. 11, 717-729) greatly reduces the accumulation of HiMet phaseolin compared to normal phaseolin. However, the increased abundance within the ER is similar for both phaseolin-HDEL and HiMet phaseolin-HDEL. Using immunocytochemistry with specific antibodies, HiMet phaseolin was found in the ER, the Golgi stack, and in transport vesicles indicating that it was transport competent. It was also present at an early stage of seed development in the protein-storage vacuoles, but was not found there at later stages of seed development. Together these results support the conclusion that the HiMet epitope did not alter the structure of the protein sufficiently to make it transport incompetent.

Heat-induced synthesis and tunicamycin-sensitive secretion of the putative storage glycoprotein conglutin gamma from mature lupin seeds

SDS/PAGE, immune blotting with specific antibodies and amino acid sequence analyses revealed that 90% of the protein released from Lupinus albus seeds incubated in water at 60 degrees C for about 3 h was conglutin gamma, a putative storage glycoprotein already present in the protein bodies of mature seeds. Incorporation of [14C]leucine into the protein demonstrated that conglutin gamma was newly synthesized during the treatment and the use of protein synthesis inhibitors ruled out the secretion of constitutive conglutin gamma. Synthesis and secretion took place only over a narrow temperature range, 57.5-62.5 degrees C, and in a short time interval, 135-180 min, of incubation of the seed. The amount of secreted conglutin gamma, i.e. 1 mg/seed, was about three times that present inside the treated or untreated seed. Secreted conglutin gamma contained covalently linked carbohydrate as well as the constitutive protein. Inhibition of the glycosylation by tunicamycin did not affect conglutin gamma synthesis, but prevented its secretion from the seed, as indicated by quantifying conglutin gamma remaining in the seed. An accumulation of the protein outside the protein bodies and at the cotyledonary cell periphery was shown in these samples by immunocytochemistry. Peptide mapping of the fragments obtained by incubation of constitutive and secreted conglutin gamma with trypsin and pepsin revealed no difference between the two proteins. Lupin seeds were still viable after the treatment. However no similarities between conglutin gamma and heat-shock proteins were observed either in the amino acid sequence or other molecular features.

Structure of the cDNA coding for conglutin gamma, a sulphur-rich protein from Lupinus angustifolius

The sequence of cDNA coding for a sulphur-rich storage protein from Lupinus angustifolius L., conglutin gamma, was determined. The coding region contained an N-terminal leader peptide of 28 amino acids which directly preceded subunits of M(r) 28,239 and 16,517. Extensive sequence homology between the protein encoded by conglutin gamma cDNA and basic 7S globulin from soybean was observed. Sequence homology to proteins from other classes of storage proteins, 11S, 7S and 2S, was limited to short and highly fragmented sequences. The amino acid sequence, Asn-Gly-Leu-Glu-Glu-Thr, characteristic of the primary site for post-translational cleavage of the precursors of 11S proteins, was absent from the sequence predicted for prepro-conglutin gamma. It is concluded that conglutin gamma is a representative of a fourth type of storage protein in legumes, distinct from the 11S, 7S and 2S storage protein families.

The legumin precursor from white lupin seed. Identity of the subunits, assembly and proteolysis

The precursors of the legumin-like storage protein from developing white lupin seeds (35 days after flowering) are trimers composed of protomers of M(r) 72,000 or 67,000. Some subunits of these oligomers contain processed precursor polypeptides, namely alpha polypeptides of either 52,000 or 44,000 linked through disulphide bonds to a beta polypeptide of 21,000, typical of the mature legumin. The prolegumin is glycosylated. Legumin oligomers purified from the same seeds are both trimers and hexamers; some of their subunits are still made of precursor polypeptides. The hexamer contains less precursor polypeptide than the trimer. A low level or absence of precursor appears to be a condition of hexamer assembly. The heterogenous prolegumin and legumin oligomers represent intermediates in the processing of the prolegumin to mature legumin. Hydrophobic-interaction chromatography on TSK-phenyl-5PW and titration with the hydrophobic probe 8-anilino-1-naphthalenesulphonate indicate that the legumin is less hydrophobic than the prolegumin. This is attributed to structural rearrangements at processing of the propolypeptide, made evident by the behaviour in CD and by the second-derivative ultraviolet spectra of the two proteins. The total protein extract of developing cotyledons at 40 days after flowering contains endopeptidases, similar to those existing in the resting seeds, which cause a limited cascade degradation of the prolegumin and legumin.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

Association and folding in legumin oligomers of lupin seed

We studied the association and conformational behavior under native or denaturing conditions in the 12S in equilibrium with 7S oligomers of lupin legumin and in the modified 7S (m7S) oligomer, which has lost the capacity to make a 12S molecule. Circular dichroism (CD), gel filtration FPLC, and PAGE were used. The native m7S oligomer has more alpha helix and nearly the same amount of beta structure as the 12S in equilibrium with 7S preparation. Conditions that shift the equilibrium in the 12S in equilibrium with 7S system toward the 7S oligomer also make the secondary structure more similar to that of m7S molecules: higher negative ellipticity appears to be a peculiarity of 7S assemblies, whether they contain modified or unmodified monomers. Part of the helical components show low stability and disappear in 1 M urea. The CD and the separation behavior on increasing the urea concentration, and in 6 M guanidine HCl, denote similar multistep unfolding in both preparations. The 12S oligomer disassembles progressively: however, also under highly denaturing conditions, modified and unmodified preparations are mainly present in an associated form. Small amounts of monomer and aggregates were detected at high denaturant concentrations.

Isolation and Characterization of Protein Bodies in Lupinus angustifolius

Using Nycodenz, a novel density gradient medium, we isolated intact protein bodies from developing seeds of Lupinus angustifolius L. (cultivar Unicrop) and achieved excellent separation from the endoplasmic reticulum, mitochondria, and other organelles. The distribution of the storage protein conglutin-beta was taken as evidence that up to 96% of the protein bodies remained intact on the gradients and banded at 1.25 grams per milliliter. The protein bodies also contained the three other abundant proteins present in L. angustifolius seeds: conglutins-alpha, -gamma, and -delta. Pulse labeling experiments were carried out to determine the site of proteolytic processing of conglutin-alpha, a legumin-like 11Svedberg unit storage protein. Cotyledons aged either 33 or 40 days after flowering were pulsed with [(3)H]leucine. Protein bodies obtained from the cotyledons aged 33 days after flowering contained only the labeled precursors of conglutin-alpha with molecular weights 85,000, 72,000, and 64,000, even after a 4 hour chase of the radioactivity. Protein bodies obtained from the cotyledons aged 40 days after flowering contained the same radioactive precursors if the tissue had been pulsed for 2 hours, and the processing products of these precursors when the tissue had been chased for 4 hours. These studies confirm that the subcellular location of proteolytic cleavage of this legumin-like protein is the protein body, that this activity is detected only in protein bodies from lupin seeds aged between 33 and 40 days of seed development after flowering and that protein bodies from seeds younger than this contain only unprocessed conglutin-alpha.