Tim23 links the inner and outer mitochondrial membranes

Tim23, a key component of the mitochondrial preprotein translocase, is anch ored in the inner membrane by its C-terminal domain and exposes an intermed iate domain in the intermembrane space that functions as a presequence rece ptor. We show that the N-terminal domain of Tim23 is exposed on the surface of the outer membrane. The two-membrane-spanning topology of Tim23 is a no vel characteristic in membrane biology. By the simultaneous integration int o two membranes, Tim23 forms contacts between the outer and inner mitochond rial membranes. Tethering the inner membrane translocase to the outer membr ane facilitates the transfer of precursor proteins from the TOM complex to the TIM23 complex and increases the efficiency of protein import.