Biochemical Consequences of Deafness-causing Actin Mutations

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The hair cell, located in the inner ear, is an essential component of the hearing process. The actin cytoskeleton plays a major role in hair cell function, through both the actin filament bundle in the stereocilium and the actin web in the cuticular plate. Six mutations have been found in gamma nonmuscle actin (DFNA 20/26) that cause autosomal dominant non-syndromic hearing loss. However, the molecular basis for the deafness caused by these mutations is unknown, in large part because of the inability to secure enough of the mutant actins to assess the effects of the mutations on actin function at the biochemical level. To address this problem and to understand the biochemical consequences of these mutations, we have cloned each mutation into yeast actin which is 91% identical to human gamma nonmuscle actin, and we can purify sufficient quantities of each of the mutant actins for biochemical analysis. These mutations cause allelespecific effects in yeast, but only one of the mutations affects polymerization of pure actin in vitro. This result suggest that the mutations may interfere, instead with actin's ability to be controlled by different actin filament regulatory proteins. We will use well-established biochemical and fluorescence microscopic assays of actin polymerization to assess the effects of these six deafness-causing mutations on actin's interaction with actinbinding proteins likely to to regulate actin filament function in the hair cell. Specifically, we will examine the mutations' effects on cofilin, an actin.filament severing protein, myosin which trafficks actin monomers and other proteins up the stereocilium, Arp2/3 complex and formins which initiate actin polymerization and control actin filament length, and espin which crosslinks actin filaments in the bundle of the stereocilium. We will also use mass spectroscopy coupled with hydrogen-deuterium exchange to determine directly the effects of the mutations on actin filament conformation and flexibility. RELEVANCE: Deafness is a major health problem in the United States. The proposed work will provide insight into the biochemical basis for the deafness caused by these six actin mutations. Additionally, the information gained on how these mutations affect actin's interaction with actin regulatory proteins will provide valuable new information concerning how actin functions and is regulated at the biochemical level in the normal ear. Thus, the work should enhance our understanding of the molecular basis of the normal hearing process.