Post-translational proteolytic processing and the isolectins of lentil and other Viciae seed lectins

Abstract

Electrospray mass spectrometry was used to identify precisely the proteolytic cleavage points within, and at the C-termini of, the proprotein forms of four Viciae lectins that give rise to their two-chain forms. The lectins examined were the pea and lentil lectins, favin and theLathyrus odoratus lectin, which represent each of the four genera in this tribe. The molecular mass data showed single β-chain forms for each lectin, with masses consistent with the available sequence and glycopeptide data, indicating that each came from a single proprotein. In contrast, the pea, lentil andL. odoratus α-chains occurred in as many as five forms, due to multiple C-terminal cleavage points. Only favin showed a single α-chain form. The α-chain mass data were again consistent with the sequence information available, except for the lenti lectin α-chain which was re-determined by protein sequencing. The two isolectin forms of this protein were shown to arise from α-chain species with and without residue Lys53. The mass spectrum of concanavalin A was also examined and both the single-chain form and the two fragment forms showed no evidence of C-terminal heterogeneity.