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 resistant, the minor dimer markedly less resistant to subtilisin than monomeric RNase A. Two S-proteins formed for each RNase A species, one starting with Ser-21, the other with Ser-22. Their relative proportions indicate that the structure of the minor dimer, whose identity with that of a RNase A dimer shown to be 3D domain-swapped is strongly suggested by recent work [S. Sorrentino et al. (2000) FEBS Lett. 466, 35-39], makes its peptide bond between Ser-21 and Ser-22 more accessible to subtilisin than it is in RNase A and its major dimer. Moreover, (i) both subunits constituting the minor dimer are more susceptible to subtilisin than monomeric RNase A, and (ii) the susceptible bonds in one of its two exchanging N-terminal arms are more accessible to the protease than in the other. The properties of the major dimer suggest that its structure could be different.