Conversion of domains into subunits in the processing of egg yolk biotin-binding protein I

Biotin-binding protein I (BBP-I), a protein that differs in its heat stability at low concentrations from that of BBP-II, has been purified from the yolk of hen oocytes and compared to BBP-II. Rabbit antiserum to BBP-II cross-reacts with identity to BBP-I. The molecular mass of BBP-I under denaturing conditions is about 68 kDa, a value four times that of BBP-II. Limited trypsin proteolysis of BBP-I generates subunits of 18 kDa with intermediate forms of approximately 51 and 34 kDa. The NH2-terminal sequence of BBP-I is very similar to that of BBP-II but has little of the polymorphism that is presumed to be generated at several positions by the slightly different subunits of BBP-II. These results indicate an unusual processing pathway in which four tandemly repeated biotin-binding domains of BBP-I become the subunits of BBP-II after limited proteolysis.