Mass spectrometric analysis of genetic and post-translational heterogeneity in the lectins Jacalin andMaclura pomifera agglutinin

Abstract

Jacalin andM. pomifera agglutinin are T-antigen specific lectins with α₄β₄ structures that show far greater microheterogeneity than plant lectins from other families, due to multiple genetic isoforms and post-translational processing. Electrospray mass spectrometry and combined liquid chromatography-electrospray mass spectrometry were used to characterize the various forms. For both lectins, the mass data were consistent with previous protein sequencing of the major α-chain species of 133 residues and three β-chain species of 20 or 21 residues. In addition, for jacalin the mass of one minor α-chain species was consistent with a second of the four reported gene sequences. However, the glycopeptide α-chain form and one β-chain form did not match any of the genes, suggesting a fifth gene remains to be found. ForM. pomifera agglutinin, three more β-chain forms were found, but all six could arise from only two genes, with additional post-translational proteolysis and post-translational substitution with an unidentified component of 106 Da creating the set of six forms. Only two α-chain forms were found also, with no glycosylation.