Two Deafness-Causing Actin Mutations (DFNA20/26) Have Allosteric Effects on the Actin Structure

Point mutations in gamma-cytoplasmic actin have been shown to result in autosomal-dominant, nonsyndromic, early-onset deafness. Two mutations at the same site, K118M and K118N, provide a unique opportunity to compare the effects of two dissimilar amino acid substitutions that produce a similar phenotype in humans. K118 resides in a helix that runs from K113 to T126, and mutations that alter the position, dynamics, and/or biochemistry of this helix can result in a wide range of pathologies. Using a combination of computational and experimental studies, both employing yeast actin, we find that these mutations at K118 result in changes in the structure and dynamics of the DNase-I loop, alterations in the structure of the H73 loop as well as the side-chain orientations of W79 and W86, changes in nucleotide exchange rates, and significant shifts in the twist of the actin monomer. Interestingly, in the case of K118N, the twist of the monomer is nearly identical to that of the F-actin protomer, and in vitro polymerization assays show that this mutation results in faster polymerization. Taken together, these results indicate that mutations at this site give rise to a series of small changes that can be tolerated in vivo but result in misregulation of actin assembly and dynamics.