Structural Basis for the Biological Activities of Bovine Seminal Ribonuclease

Abstract

Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with special biological properties that include specific antitumor, aspermatogenic, and immunosuppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys(31) of one subunit and Cys(32) Of the other. At equilibrium, this dimer is a mixture of two distinct quaternary forms, M=M and MxM. The conversion of M=M to MxM entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified MxM were shown to be greater than those of purified M=M, despite extensive equilibration of M=M and MxM during the time course of the assays. Replacing Cys(31) or Cys(32) with a serine residue did not compromise the enzymatic activity of dimeric BS-RNase, but reduced both the fraction of MxM at equilibrium and the cytotoxicity. We conclude that the MxM form is responsible for the special biological properties of BS-RNase. Since cytosolic ribonuclease inhibitor binds tightly to monomeric but not dimeric BS-RNase and only the MxM form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its MxM form to retain its lethal enzymatic activity in vivo.