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

Jacalin and M. pomifera agglutinin are T-antigen specific lectins with alpha 4 beta 4 structures that show far greater micro-heterogeneity 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 alpha-chain species of 133 residues and three beta-chain species of 20 or 21 residues. In addition, for jacalin the mass of one minor alpha-chain species was consistent with a second of the four reported gene sequences. However, the glycopeptide alpha-chain form and one beta-chain form did not match any of the genes, suggesting a fifth gene remains to be found. For M. pomifera agglutinin, three more beta-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 alpha-chain forms were found also, with no glycosylation.