Hints on the evolutionary design of a dimeric RNase with special bioactions

Residues P19, L28, C31, and C32 have been implicated with key roles in determining the dimeric structure and the N-terminal domain swapping of seminal RNase. In an attempt to have a clearer understanding of the structural and functional significance of these residues in seminal RNase, a series of mutants of pancreatic RNasAe was constructed in which one or more of the four residues were introduced into RNase A. The RNase mutantsw ere examined for: (1) the ability to form dimers;( 2) the capacity to exchange their N-terminal domains; (3) resistance to selective cleavage by subtilisin; and (4) antitumor activity. The experiments demonstrated that: (1) the presence of intersubunit disulfides is both necessary and sufficient for engenderinga stably dimeric RNase;( 2) all four residues play a role in determining the exchangeo of N-teminal domains; (3) the exchange is the molecular basis for the RNase antitumor action; and (4) this exchange is not a prerequisite in an evolutionary mechanism for the generation of dimeric RNases.