Tom7 modulates the dynamics of the mitochondrial outer membrane translocase and plays a pathway-related role in protein import.

Abstract

The preprotein translocase of the outer mitochondrial membrane is a multi‐subunit complex with receptors and a general import pore. We report the molecular identification of Tom7, a small subunit of the translocase that behaves as an integral membrane protein. The deletion of TOM7 inhibited the mitochondrial import of the outer membrane protein porin, whereas the import of preproteins destined for the mitochondrial interior was impaired only slightly. However, protein import into the mitochondrial interior was strongly inhibited when it occurred in two steps: preprotein accumulation at the outer membrane in the absence of a membrane potential and subsequent further import after the re‐establishment of a membrane potential. The delay of protein import into tom7delta mitochondria seemed to occur after the binding of preproteins to the outer membrane receptor sites. A lack of Tom7 stabilized the interaction between the receptors Tom20 and Tom22 and the import pore component Tom40. This indicated that Tom7 exerts a destabilizing effect on part of the outer membrane translocase, whereas Tom6 stabilizes the interaction between the receptors and the import pore. Synthetic growth defects of the double mutants tom7delta tom20delta and tom7delta tom6delta provided genetic evidence for the functional relationship of Tom7 with Tom20 and Tom6. These results suggest that (i) Tom7 plays a role in sorting and accumulation of the preproteins at the outer membrane, and (ii) Tom7 and Tom6 perform complementary functions in modulating the dynamics of the outer membrane translocase.

Insertion and Assembly of Human Tom7 into the Preprotein Translocase Complex of the Outer Mitochondrial Membrane

“…A BLAST search of the nucleotide data base revealed a human cDNA with significant homology (47% similarity and 28% identity) with yeast Tom7 (18). The human cDNA contains a 165-bp ORF encoding a protein of 55 amino acids with a predicted molecular mass of 6.2 kDa.…”

“…The human cDNA contains a 165-bp ORF encoding a protein of 55 amino acids with a predicted molecular mass of 6.2 kDa. An amino acid alignment of the human ORF with known Tom7 orthologs from the yeast S. cerevisiae, (18), N. crassa (39), and potato (Solanum tuberosum (40)) along with putative Tom7 proteins from Caenorhabditis elegans (GenBank TM accession number P34660) and mouse (Mus musculus, GenBank TM accession number W74865) revealed that they all contain the conserved motif aGaX-PXXXXXG where a is an aromatic residue and X is any residue (Fig. 1A).…”

“…Two additional methionines were engineered at the C terminus of Tom7 to enhance detection of the radiolabeled product (18). In vitro translation of the RNA transcripts was performed according to Ryan et al (32).…”

“…Tom6 is important for promoting the interactions between Tom20 and Tom70 with the TOM complex (17) and also for stimulating the assembly of Tom22 with Tom40 (15). In contrast, Tom7 seems to play a role in destabilizing the interactions between the TOM complex and the TOM receptors, and this may be important for insertion of proteins into the outer membrane (15,18).…”

Protein Import Into Mitochondria

“…Tom5 was initially only identi ed in Saccharomyces cerevisiae, but recently has also been detected in Neurospora crassa (Neupert et al, unpublished). Tom6 and Tom7 seem to be involved in the regulation of the assembly and disassembly of receptor proteins with the GIP, a process that is probably mediated by Tom22 (41,48,(54)(55)(56)(57)(58). Recently it was shown in cross-linking experiments that Neurospora crassa Tom6 and Tom7 are in vicinity of preproteins, which are in transit across the outer membrane (58).…”