Prestin is believed to be the motor protein of outer hair cells. In this study, characterization of prestin was performed by mutational analysis focusing on the unique amino acids in prestin. Five prestin mutants, namely, M1221, C192A, M225Q, C415A and T428L, were engineered to be separately expressed in HEK293 cells. M225Q exhibited 1.7 times greater nonlinear capacitance (NLC), which reflects the charge transfer of prestin in the plasma membrane, than that of wild-type prestin (WT), although other mutants showed NLC similar to that of WT, indicating that M122, C192, C415 and T428 are not involved in the charge transfer of prestin. On the other hand, the amount of M225Q molecules in the plasma membrane was 1.2 times greater than that of WT molecules. The increase in the amount of prestin molecules by the mutation in M225 probably indicates that prestin became more stable, but such increase does not correspond to the increase of NLC. Hence, the mutation in M225 was considered to make prestin more stable, leading to an increase in the ratio of active prestin molecules to the total amount of prestin molecules in the plasma membrane.
