PROTEOLYSIS OF PHASEOLIN IN RELATION TO ITS STRUCTURE

Phaseolin differs from other native legume proteins in that its hydrol ysis by trypsin and pepsin stops after a limited number of peptides ha ve been cleaved off. Concomitantly, trypsin splits each phaseolin subu nit approximately into halves. The N-terminal sequencing of these fina l hydrolysis products of high molecular mass showed loss of a tetrapep tide from the N terminus of each subunit, with a second cleavage in th e interdomain linker. Other probable sites cleaved by trypsin, which a ccount for the quantity of degraded protein, may be deduced from the t ertiary structure of phaseolin and from the specificity of trypsin. Pr oteolysis by pepsin is limited to cleavage of seven amino acids from t he N terminus, and of two to three peptides probably from the disorder ed C-terminal segment of phaseolin subunits. The cleavage site in the N-terminal sequence has been identified. The peculiarities of the phas eolin structure that may cause its resistance to the proteolytic attac k are discussed.