The synthesis and structure of pea storage proteins

Abstract

The seeds of pea, Pisum sativum L., like those of other legumes, accumulate proteins in their cotyledons during development. These proteins are mainly globulins, soluble in aqueous salt solutions at neutral pH; in keeping with their physiological storage role they are multimeric proteins readily rendered insoluble for deposition, and have high proportions of amide amino acids. They are degraded on germination of the seed to provide nutrients for the growing plant. The two major seed proteins in pea are legumin, a hexameric molecule (M_r ⋍ 380,000) where each monomer (M_r ⋍ 60,000) contains an “acidic”; (M_r ⋍ 40,000) and a “basic”; (M_r ⋍ 20,000) subunit linked by a disulphide bond, and vicilin, a trimeric molecule (M_r ⋍ 150,000) where each monomer (M_r ⋍ 50,000) may contain breaks in its polypeptide chain, giving rise to a variety of smaller subunits. Both these proteins are synthesised on polyribosomes bound to the endoplasmic reticulum in cotyledon cells and are transported from there to membrane‐enclosed protein bodies for deposition. Legumin and vicilin are both modified by post‐translational processing, primarily specific proteolysis, after synthesis in vivo. The major seed storage proteins in pea are produced from mRNA species that are only detectable in seed tissues, and that are transcribed from a limited number (3 to 10) of single copy genes. Regulation of the synthesis of these proteins is determined primarily by mRNA levels, which are in turn determined by separable “switch‐on”; events at the level of gene transcription, alterations in overall transcriptional activity during seed development, and the stability of the mRNA species themselves.