ASSIGNMENT AND SECONDARY-STRUCTURE DETERMINATION OF MONOMERIC BOVINE SEMINAL RIBONUCLEASE EMPLOYING COMPUTER-ASSISTED EVALUATION OF HOMONUCLEAR 3-DIMENSIONAL H-1-NMR SPECTRA

Monomeric bovine seminal ribonuclease (mBS-RNase), the subunit of dime ric bovine seminal ribonuclease (BS-RNase), is an unusual monomer: for its structural stability, its catalytic activity, which is even highe r than that of the parent dimeric enzyme, and for its role as an inter mediate in the refolding of dimeric BS-RNase. Here we present the prot on NMR assignment and secondary-structure determination of mBS-RNase, with a comparison of its structure to the structure of its parent prot ein, and to the structure of RNase A, a homologue with more than 80% i dentity in amino acid sequence. Proton NMR assignment was performed us ing a computer-assisted procedure, through a partially automated analy sis of homonuclear three-dimensional spectra [Oschkinat, H., Holak, T. A. and Cieslar, C. (1991) Biopolymers 31, 699-712]. The secondary str uctures of mBS-RNase, of the A chain of dimeric BS-RNase, and of RNase A, are found to be similar. Significant differences are found instead , between mBS-RNase and RNase A in the more flexible stretches of the molecule, where a higher number of substitutions is present. Furthermo re, a preliminary tertiary-structure model is reported, showing that t he overall folding of mBS-RNase is closer to that of RNase A rather th an that of (dimeric) BS-RNase.