Tim18p is a new component of the Tim54p-Tim22p translocon in the mitochondrial inner membrane

The mitochondrial inner membrane contains two separate translocons: one req uired for the translocation of matrix-targeted proteins (the Tim23p-Tim17p complex) and one for the insertion of polytopic proteins into the mitochond rial inner membrane (the Tim54p-Tim22p complex). To identify new members of the Tim54p-Tim22p complex, we screened for high-copy suppressors of the te mperature-sensitive tim54-1 mutant. We identified a new gene, TIM18, that e ncodes an integral protein of the inner membrane. The following genetic and biochemical observations suggest that the Tim18 protein is part of the Tim 54p-Tim22p complex in the inner membrane: multiple copies of TIM18 suppress the tim54-1 growth defect; the tim18::HIS3 disruption is synthetically let hal with tim54-1; Tim54p and Tim22p can be coimmune precipitated with the T im18 protein; and Tim18p, along with Tim54p and Tim22p, is detected in an s imilar to 300-kDa complex after blue native electrophoresis. We propose tha t Tim18p is a new component of the Tim54p-Tim22p machinery that facilitates insertion of polytopic proteins into the mitochondrial inner membrane.