In vitro translation and processing of a precursor form of favin, a lectin from Vicia faba

Favin, the glucose- and mannose-binding lectin isolated from fava (Vicia faba) beans, consists of two polypeptide chains (alpha, Mr = 5,571; beta, Mr = 20,700). Translation of fava bean mRNA in vitro in a wheat germ-derived system yields a single favin polypeptide chain of Mr = 29,000. This molecule appears to consist of a hydrophobic 29-amino acid residue signal sequence at the NH2 terminus followed by the beta chain sequence; it also includes the alpha chain sequence. These results suggest that the alpha and beta chains arise by post-translational cleavage of a single precursor polypeptide: signal-beta chain-alpha chain. The signal peptide is similar in sequence to those seen in animal and prokaryotic systems, suggesting that translocation mechanisms are highly conserved. Translation of favin mRNA in the presence of dog pancreas microsomal membranes yields at least three polypeptides in addition to the presumed precursor chain. The largest of these molecules is translocated into the lumen of the membrane vesicles and glycosylated but its signal sequence remains intact. The two other species are translocated and glycosylated, but their signal sequences have been removed; they appear to differ from each other in that one begins with the beta chain sequence and the other begins one residue after the NH2-terminal threonine of the beta chain. These three variants could reflect normal features of the processing of the favin precursor but more likely result from aberrant processing of the plant protein by dog pancreas membranes.