Molecular Effects of Deafness Mutations in Actin

Abstract

Point mutations in γ-cytoplasmic actin have been shown to cause autosomal-dominant non-syndromic early onset deafness. Of the eleven known actin mutations, two are unique in that they occur in the same residue. The mutations, K118M in one family and K118N in a second family, 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 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 using 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 sidechain orientations of W79 and W86, changes in nucleotide exchange rates, and significant shifts in the twist of the actin monomer. Interestingly for the case of K118N, the twist of the monomer is nearly identical to that for the F-actin protomer, and in vitro polymerization assays show that this mutation actually results in faster polymerization. Taken together, it is evident that mutations at this site give rise to a series of smaller changes that can be tolerated in vivo, but result in misregulation of actin assembly and dynamics.