Genetic and structural characterization of the human mitochondrial inner membrane translocase

Translocation of nuclear-encoded mitochondrial preproteins is mediated by t ranslocases in the outer and inner membranes. In the yeast Saccharomyces ce revisiae, translocation of preproteins into the matrix requires the membran e proteins Tim23, Tim17 and Tim44, which drive translocation in cooperation with mtHsp70 and its co-chaperone Mge1p. We have cloned and functionally a nalyzed the human homologues of Tim17, Tim23 and Tim44. In contrast to yeas t, two TIM17 genes were found to be expressed in humans. TIM44, TIM23 and T IM17a genes were mapped to chromosomes 19p13.2-p13.3, 10q11.21-q11.23 and 1 q32. The TIM17b gene mapped to Xp11.23, near the fusion point where an auto somal region was proposed to have been added to the "ancient" part of the X chromosome about 80-130 MY ago. The primary sequences of the two proteins, hTim17a and hTim17b, are essentially identical, significant differences be ing restricted to their C termini. They are ubiquitously expressed in fetal and adult tissues, and both show expression levels comparable to that of h Tim23. Biochemical characterization of the human Tim components revealed th at hTim44 is localized in the matrix and, in contrast to yeast, only loosel y associated with the inner membrane. hTim23 is organized into two distinct complexes in the inner membrane, one containing hTim17a and one containing hTim17b. Both TIM complexes display a native molecular mass of 110 kDa. We suggest that the structural organization of TIM23 . 17 preprotein transloc ases is conserved from low to high eukaryotes. (C) 1999 Academic Press.