Structural basis for the biological activities of bovine seminal ribonuclease.
Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with special biological properties that include specific antitumor, aspermatogenic, and immuno-suppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys31 of one subunit and Cys32 of the other. At equilibrium, this dimer is a mixture of two distinct quaternary forms, M = M and M x M. The conversion of M = M to M x M entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified M x M were shown to be greater than those of purified M = M, despite extensive equilibration of M = M and M x M during the time course of the assays. Replacing Cys31 or Cys32 with a serine residue did not compromise the enzymatic activity of dimeric BS-RNase, but reduced both the fraction of M x M at equilibrium and the cytotoxicity. We conclude that the M x M 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 M x M form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its M x M form to retain its lethal enzymatic activity in vivo.
Kim
J S
JS
Department of Biochemistry, University of Wisconsin, Madison 53706-1569, USA.
Soucek
J
J
Matousek
J
J
Raines
R T
RT
D000818:Animals; D001483:Base Sequence; D002417:Cattle; D017931:DNA Primers / Q000737:chemistry; D006131:Growth Inhibitors; D066298:In Vitro Techniques; D007700:Kinetics; D008213:Lymphocyte Activation / Q000187:drug effects; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012260:Ribonucleases / Q000737:chemistry*; D012661:Semen / Q000201:enzymology; D013091:Spermatogenesis / Q000187:drug effects; D013329:Structure-Activity Relationship; D013816:Thermodynamics; D014407:Tumor Cells, Cultured / Q000166:cytology
D017931:DNA Primers; D006131:Growth Inhibitors; D012260:Ribonucleases