Structural versatility of bovine ribonuclease A - Distinct conformers of trimeric and tetrameric aggregates of the enzyme

Lyophilization of bovine ribonuclease A (RNase A; Sigma, type XII-A) from 4 0% acetic acid solutions leads to the formation of approximate to 14 aggreg ated species that can be separated by ion-exchange chromatography. Several aggregates were identified, including two variously deamidated dimeric subs pecies, two distinct trimeric and two distinct tetrameric RNase A conformer s, besides the two forms of dimer characterized previously [Gotte, G. & Lib onati, M. (1998) Two different forms of aggregated dimers of ribonuclease A . Biochim. Biophys. Acta 1386, 106-112]. We also have possible evidence for the existence of two forms of pentameric RNase A. The two forms of trimers and tetramers are characterized by: (a) slightly different gel filtration patterns; (b) different retention times in ion-exchange chromatography; and (c) different mobilities in cathodic gel electrophoresis under nondenaturi ng conditions. Therefore, they appear to have distinct structural organizat ions responsible for a different availability of their positively charged a mino acid residues. All RNase A oligomers, in particular the two distinct t rimeric and tetrameric conformers, degrade poly(A) poly(U), viral double-st randed RNA and polyadenylate with a catalytic efficiency that is in general higher for the more basic species. On the contrary, the activity of the RN ase A oligomers, from dimer to pentamer, on yeast RNA and poly(C) (Kunitz a ssay) is lower than that of monomeric RNase A.