Identification and characterisation of seed storage protein transcripts from Lupinus angustifolius

BACKGROUND

In legumes, seed storage proteins are important for the developing seedling and are an important source of protein for humans and animals. Lupinus angustifolius (L.), also known as narrow-leaf lupin (NLL) is a grain legume crop that is gaining recognition as a potential human health food as the grain is high in protein and dietary fibre, gluten-free and low in fat and starch.

RESULTS

Genes encoding the seed storage proteins of NLL were characterised by sequencing cDNA clones derived from developing seeds. Four families of seed storage proteins were identified and comprised three unique α, seven β, two γ and four δ conglutins. This study added eleven new expressed storage protein genes for the species. A comparison of the deduced amino acid sequences of NLL conglutins with those available for the storage proteins of Lupinus albus (L.), Pisum sativum (L.), Medicago truncatula (L.), Arachis hypogaea (L.) and Glycine max (L.) permitted the analysis of a phylogenetic relationships between proteins and demonstrated, in general, that the strongest conservation occurred within species. In the case of 7S globulin (β conglutins) and 2S sulphur-rich albumin (δ conglutins), the analysis suggests that gene duplication occurred after legume speciation. This contrasted with 11S globulin (α conglutin) and basic 7S (γ conglutin) sequences where some of these sequences appear to have diverged prior to speciation. The most abundant NLL conglutin family was β (56%), followed by α (24%), δ (15%) and γ (6%) and the transcript levels of these genes increased 103 to 106 fold during seed development. We used the 16 NLL conglutin sequences identified here to determine that for individuals specifically allergic to lupin, all seven members of the β conglutin family were potential allergens.

CONCLUSION

This study has characterised 16 seed storage protein genes in NLL including 11 newly-identified members. It has helped lay the foundation for efforts to use molecular breeding approaches to improve lupins, for example by reducing allergens or increasing the expression of specific seed storage protein(s) with desirable nutritional properties.

Characterization of a tomato protein that inhibits a xyloglucan-specific endoglucanase

A basic, 51 kDa protein was purified from suspension-cultured tomato and shown to inhibit the hydrolytic activity of a xyloglucan-specific endoglucanase (XEG) from the fungus Aspergillus aculeatus. The tomato (Lycopersicon esculentum) protein, termed XEG inhibitor protein (XEGIP), inhibits XEG activity by forming a 1 : 1 protein:protein complex with a Ki approximately 0.5 nm. To our knowledge, XEGIP is the first reported proteinaceous inhibitor of any endo-beta-1,4-glucanase, including the cellulases. The cDNA encoding XEGIP was cloned and sequenced. Database analysis revealed homology with carrot extracellular dermal glycoprotein (EDGP), which has a putative role in plant defense. XEGIP also has sequence similarity to ESTs from a broad range of plant species, suggesting that XEGIP-like genes are widely distributed in the plant kingdom. Although Southern analysis detected only a single XEGIP gene in tomato, at least five other XEGIP-like tomato sequences have been identified. Similar small families of XEGIP-like sequences are present in other plants, including Arabidopsis. XEGIP also has some sequence similarity to two previously characterized proteins, basic globulin 7S protein from soybean and conglutin gamma from lupin. Several amino acids in the XEGIP sequence, notably 8 of the 12 cysteines, are generally conserved in all the XEGIP-like proteins we have encountered, suggesting a fundamental structural similarity. Northern analysis revealed that XEGIP is widely expressed in tomato vegetative tissues and is present in expanding and maturing fruit, but is downregulated during ripening.

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.