The biosynthesis and primary structure of pea seed lectin

Isolation of pea lectin from immature seeds, together with in vivo pulse-chase labeling and in vitro translation of RNA from such seeds, all gave results which indicated strongly that pea lectin is initially synthesized as a large precursor (pre-pro-form) (Mr = 25,000) which is cleaved both co- and post-translationally to yield the alpha and beta subunits of mature lectin. Two overlapping cDNA plasmids complementary to lectin mRNA were sequenced. They coded for both beta and alpha subunits of lectin, in the orientation beta leads to alpha from the NH2 terminus, together with a hydrophobic NH2-terminal leader sequence and included a 3' untranslated region of 124 nucleotides. In the appropriate reading frame, no stop codon was found between the coding sequences for the beta and alpha subunits, confirming the synthesis of pea lectin via a pre-pro-form. The entire pea lectin sequence showed a high degree of homology with the published amino acid sequences of lectins from lentil, broad bean, and, to a lesser extent, with concanavalin A from jack bean. Sequence variation occurred mainly in those regions thought not to be involved in metal- and sugar-binding or in the formation of beta-pleated sheets.