The secondary structure of a pyrimidine-guanine sequence-specific ribonuclease possessing cytotoxic activity from the oocytes of Rana catesbeiana

RC-RNase is a pyrimidine-guanine sequence-specific ribonuclease and a sialic-acid-binding lectin purified from Rana catesbeiana (bullfrog) oocytes. This 111-amino acid protein exhibits cytotoxicity toward several tumor cell lines. In this paper we report the assignments of proton NMR resonances and the identification of the secondary structure deduced from NOE constraints, chemical shift index, ³J_NHα and amide proton exchange rates. The protein was directly isolated from bullfrog oocytes; we were able to assign all but five of the amino acid backbone protons of the unlabeled protein by analyzing a large set of two-dimensional proton NMR spectra obtained at several temperatures and pH conditions. Our results indicate that the structure of RC-RNase is dominated by the presence of two triple-stranded anti-parallel β-sheets and three α-helices, similar to those of the pyrimidine family ribonucleases. Two sets of resonances were observed for 11 amide protons and 8 α-protons located in the loop-1 region, an α2 helix, and three β-strands (β1, β3 and β4), suggesting the presence of nonlocalized multiple conformations for RC-RNase.