BIOGENESIS OF TIM23 AND TIM17, INTEGRAL COMPONENTS OF THE TIM MACHINERY FOR MATRIX-TARGETED PREPROTEINS

We analysed the import pathway of Tim23 and of Tim17, components of th e mitochondrial import machinery for matrix-targeted preproteins, Tim2 3 contains two independent import signals, One is located within the f irst 62 amino acid residues of the hydrophilic domain that, in the ass embled protein, is exposed to the intermembrane space, This signal med iates translocation of Tim23 across the outer membrane independently o f the membrane potential, Delta Psi. A second import signal is located in the C-terminal membrane-integrated portion of Tim23, It mediates t ranslocation across the outer membrane and insertion into the inner me mbrane in a strictly Delta Psi-dependent fashion, Structurally, Tim17 is related to Tim23 but lacks a hydrophilic domain, It contains an imp ort signal in the C-terminal half and its import requires Delta Psi. T he Delta Psi-dependent import signals of Tim23 and Tim17 are located a t corresponding sites in these two homologous proteins, They exhibit f eatures reminiscent of the positively charged N-terminal presequences of matrix-targeted precursors, Import of Tim23 and its insertion into the inner membrane requires Tim22 but not functional Tim23, Thus, biog enesis of the Tim23.17 complex depends on the Tim22 complex, which is the translocase identified as mediating the import of carrier proteins .