Identification of beta-actin as a candidate autoantigen in autoimmune inner ear disease

It has been shown that sera from patients with autoimmune inner ear disease contain antibodies to several inner ear antigens. We report here the chara cterization of the 42-43 kDa protein against which a significant number of patients' sera react strongly. After separation of inner ear proteins from guineapig cochleas by SDS-PAGE, the band corresponding to the 42-43 kDa pro tein was digested with trypsin and the peptide fragments were separated by high-performance liquid chromatography. Two fractions were then subjected t o amino acid sequencing by the classical automated Edman degradation. The s equence of a stretch of 15 amino acids of the first fragment was identical to that of amino acids 148-162 of beta -actin. The sequence of the 10 amino acids of the second fragment was also identical to beta -actin. On Western blots, monoclonal antibody directed against beta -actin reacted with the i nner ear 42-43 kDa proteins. The serum samples from the patients and the mo noclonal antibody reacted with the nonmuscle actin used as antigen in Weste rn blotting. Immunoblot analysis of inner ear proteins after two-dimensiona l gel electrophoresis showed a spot, corresponding to the region of the 43 kDa as compared to the protein standards. On the basis of these data it is concluded that the target 42-43 kDa protein for antibodies in sera of patie nts with autoimmune inner ear disease is beta -actin, a molecule, which has important and numerous functions inside cells. This is the first report to identify the cytoskeletal protein beta -actin as a candidate autoantigen i n autoimmune inner ear disease.