Dimer formation by a "monomeric" protein

Dimeric proteins can arise by the swapping of structural domains between mo nomers. The prevalence of this occurrence is unknown. Ribonuclease A (RNase A) is assumed to be a monomer near physiological conditions. Here, this hy pothesis is tested and found to be imprecise. The two histidine residues (H is12 and His119) in the active site of RNase A arise from two domains (S-pe ptide and S-protein) of the protein. The H12A and H119A variants have 10(5) -fold less ribonucleolytic activity than does the wild-type enzyme. Incubat ing a 1.1 mixture of the H12A and H119A variants at pH 6.5 and 65 degreesC results in a 10(3)-fold increase in ribonucleolytic activity. A large quant ity of active dimer can be produced by lyophilizing a 1:1 mixture of the H1 2A and H119A variants from acetic acid. At pH 6.5 and 65 degreesC, the ribo nucleolytic activity of this: dimer converges to that of the dimer formed b y simply incubating the monomers, as expected for a monomer-dimer equilibri um. The equilibrium dissociation constant for the dimer is near 2, mM at bo th 65 and 37 degreesC. This value of K-d is only 20-fold greater than the c oncentration of RNase A in the cow pancreas, suggesting that RNase A dimers exist in vivo. The intrinsic ability of RNase A to form dimers under physi ological conditions is consistent with a detailed model for the evolution o f homodimeric proteins. Dimers of "monomeric" proteins could be more preval ent than is usually appreciated.