The structure and function of actin in hair cells.

Within the last 7 years, the protein actin has been identified in the stereocilia and cuticular plate region of the hair cell. An intensive effort has been mounted to describe the structural organization of this protein, to identify actin-associated proteins in these regions, and to identify the functional role of these proteins. The paracrystalline array of actin and its cross bridges imparts rigidity and stiffness to the stereocilia, which are important in determining their response properties. It also appears that these properties can be changed if the paracrystalline array is damaged by noise exposure. The functional implications of stereocilia rigidity and stiffness, as well as potential contractile mechanisms in the hair cell, are discussed. Finally, it is suggested that changes in the cross-sectional shape of the stereocilia caused by shearing of actin filaments during stereocilia deflections can be related to the mechano-electrical events in the plasma membrane of the cell. This may be the link between the transmission of vibrational energy through the sensory accessory structures of the peripheral ear, and the initiation of electrochemical events associated with transduction.