Tim18p, a new subunit of the TIM22 complex that mediates insertion of imported proteins into the yeast mitochondrial inner membrane

Import of carrier proteins from the cytoplasm into the mitochondrial inner membrane of yeast is mediated by a distinct system consisting of two solubl e 70-kDa protein complexes in the intermembrane space and a 300-kDa complex in the inner membrane, the TIM22 complex. The TIM22 complex contains the p eripheral subunits Tim9p, Tim10p, and Tim12p and the integral membrane subu nits Tim22p and Tim54p. We identify here an additional subunit, an 18-kDa i ntegral membrane protein termed Tim18p. This protein is made as a 21.9-kDa precursor which is imported into mitochondria and processed to its mature f orm. When mitochondria are gently solubilized, Tim18p comigrates with the o ther subunits of the TIM22 complex on nondenaturing gels and is coimmunopre cipitated with Tim54p and Tim12p. Tim18p does not cofractionate with the TI M23 complex upon immunoprecipitation or nondenaturing gel electrophoresis. Deletion of Tim18p decreases the growth rate of yeast cells by a factor of two and is synthetically: lethal with temperature-sensitive mutations in Ti m9p or Tim10p. It also impairs the import of several precursor proteins int o isolated mitochondria, and lowers the apparent mass of the TIM22 complex. We suggest that Tim18p functions in the assembly and stabilization of the TIM22 complex but does not directly participate in protein insertion into t he inner membrane.