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Dimer formation by a “monomeric” protein
Author(s) -
Park Chiwook,
Raines Ronald T.
Publication year - 2000
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1110/ps.9.10.2026
Subject(s) - dimer , ribonuclease , rnase p , monomer , chemistry , bovine pancreatic ribonuclease , dissociation constant , histidine , s tag , stereochemistry , enzyme , dissociation (chemistry) , rnase ph , protein engineering , biochemistry , rna , organic chemistry , receptor , gene , polymer
Dimeric proteins can arise by the swapping of structural domains between monomers. The prevalence of this occurrence is unknown. Ribonuclease A (RNase A) is assumed to be a monomer near physiological conditions. Here, this hypothesis is tested and found to be imprecise. The two histidine residues (His12 and His119) in the active site of RNase A arise from two domains (S‐peptide and S‐protein) of the protein. The H12A and H119A variants have 10 5 ‐fold less ribonucleolytic activity than does the wild‐type enzyme. Incubating a 1:1 mixture of the H12A and H119A variants at pH 6.5 and 65°C results in a 10 3 ‐fold increase in ribonucleolytic activity. A large quantity of active dimer can be produced by lyophilizing a 1:1 mixture of the H12A and H119A variants from acetic acid. At pH 6.5 and 65°C, the ribonucleolytic activity of this dimer converges to that of the dimer formed by simply incubating the monomers, as expected for a monomer–dimer equilibrium. The equilibrium dissociation constant for the dimer is near 2 mM at both 65 and 37°C. This value of K d is only 20‐fold greater than the concentration 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 physiological conditions is consistent with a detailed model for the evolution of homodimeric proteins. Dimers of “monomeric” proteins could be more prevalent than is usually appreciated.