Differential Expression of Seed Storage Protein Genes in the Pea legJ Subfamily; Sequence of Gene legK

Towards absolute quantification of protein genetic variants in Pisum sativum extracts

In recent years, several studies have used proteomics approaches to characterize genetic variant profiles of agricultural raw materials. In such studies, the challenge is the quantification of the individual protein variants. In this study a novel UPLC-PDA-MS method with absolute and label-free UV-based peptide quantification was applied to quantify the genetic variants of legumin, vicilin and albumins in pea extracts. The aim was to investigate the applicability of this method and to identify challenges in determining protein concentration from the measured peptide concentrations. Analysis of the protein mass balance showed significant losses of proteins in extraction (37%) and of peptides in further sample preparation (69%). The challenge in calculating the extractable individual protein concentrations was how to deal with these insoluble peptides. The quantification approach using average amino acid concentrations in each position of the sequence showed most reproducible results and allowed comparison of the genetic protein composition of 8 different cultivars. The extractable protein composition (μM/μM) was remarkably similar for all cultivar extracts and consisted of legumins A1 (12.8 ± 1.2%), A2 (1.1 ± 0.4%), B (9.9 ± 1.6%), J (7.5 ± 1.0%) and K (10.3 ± 2.1%), vicilin (15.2 ± 1.7%), provicilin (15.7 ± 2.5%), convicilin (9.8 ± 0.8%), albumin A1 (7.4 ± 2.0%), albumin 2 (10.0 ± 1.5%) and protease inhibitor (0.4 ± 0.4%).

Vicilin and convicilin are potential major allergens from pea

BACKGROUND

Allergic reactions to pea (Pisum sativum) ingestion are frequently associated with lentil allergy in the Spanish population. Vicilin have been described as a major lentil allergen.

OBJECTIVE

To identify the main IgE binding components from pea seeds and to study their potential cross-reactivity with lentil vicilin.

METHODS

A serum pool or individual sera from 18 patients with pea allergy were used to detect IgE binding proteins from pea seeds by immunodetection and immunoblot inhibition assays. Protein preparations enriched in pea vicilin were obtained by gel filtration chromatography followed by reverse-phase high-performance liquid chromatography (HPLC). IgE binding components were identified by means of N-terminal amino acid sequencing. Complete cDNAs encoding pea vicilin were isolated by PCR, using primers based on the amino acid sequence of the reactive proteins.

RESULTS

IgE immunodetection of crude pea extracts revealed that convicilin (63 kDa), as well as vicilin (44 kDa) and one of its proteolytic fragments (32 kDa), reacted with more than 50% of the individual sera tested. Additional proteolytic subunits of vicilin (36, 16 and 13 kDa) bound IgE from approximately 20% of the sera. The lentil vicilin allergen Len c 1 strongly inhibited the IgE binding to all components mentioned above. The characterization of cDNA clones encoding pea vicilin has allowed the deduction of its complete amino acid sequence (90% of sequence identity to Len c 1), as well as those of its reactive proteolytic processed subunits.

CONCLUSIONS

Vicilin and convicilin are potential major allergens from pea seeds. Furthermore, proteolytic fragments from vicilin are also relevant IgE binding pea components. All these proteins cross-react with the major lentil allergen Len c 1.

Genetic aspects of the organization of legumin genes in pea

We have compared physical and genetic maps of the region around the legJ gene in pea. In this vicinity there are four B-type legumin genes, arranged as two close pairs. The detection of a recombination event within this gene cluster allows the orientation of this group of genes within the surrounding linkage group to be determined. The relationship between physical and genetic distances in this region is discussed, as are the implications of this for relating physical and genetic maps elsewhere in the pea genome.