Deglycosylation is necessary but not sufficient for activation of proconcanavalin A

Concanavalin A (ConA), one of the most studied plant lectins, is formed in jack bean (Canavalia ensiformis) seeds. ConA is synthesized as an inactive glycoprotein precursor proConA, Different processing events such as endopro teolytic cleavages, ligation of peptides and deglycosylation of the precurs or are required to generate the different polypeptides constitutive of matu re ConA, Among these events, deglycosylation of the prolectin appears as a key step in the lectin activation. The detection of deglycosylated proConA in immature jack bean seeds indicates that endoproteolytic cleavages are no t prerequisite for its deglycosylation, Both the structure of the lectin pr ecursor N-glycans Man(8-9)GlcNAc(2) and the capacity of Endo H to cleave th ese oligosaccharide from native proConA in vitro favoured Endo H-type glyco sidases as candidates for proConA deglycosylation in planta. Evidence for p H-dependent changes in the prolectin folding were obtained from analysis of the N-glycan accessibility and activation of the deglycosylated lectin pre cursor in acidic conditions. These data are consistent with the observation that both deglycosylation and acidification of the pH are the minimum requ irements to convert the inactive precursor into an active lectin.