Different susceptibility of the two dimers of ribonuclease A to subtilisin. Implications for their structure

RNase A and its minor and major dimers were digested with subtilisin under controlled conditions. The major dimer was found to be slightly more resist ant, the minor dimer markedly less resistant to subtilisin than monomeric R Nase A. Two S-proteins formed for each RNase A species, one starting with S er-21 the other with Ser-22. Their relative proportions indicate that the s tructure of the minor dimer, whose identity with that of a RNase A dimer sh own to be 3D domain-swapped is strongly suggested by recent work [S. Sorren tino et al. (2000) FEES Lett. 466, 35-39], makes its peptide bond between S er-21 and Ser-22 more accessible to subtilisin than it is in RNase A and it s major dimer. Moreover, (i) both subunits constituting the minor dimer are more susceptible to subtilisin than monomeric RNase A, and (ii) the suscep tible bonds in one of its two exchanging N-terminal arms are more accessibl e to the protease than in the other. The properties of the major dimer sugg est that its structure could be different. (C) 2001 Elsevier Science B.V. A ll rights reserved.