Post Receptor Import Step

Functional studies have shown that mitochondrial proteins may use different receptor systems but then pass a common channel, named general import pore (GIP). This GIP guides polypeptides through the OMM and is thought to be formed by the Tom40 protein that is deeply embedded in the membrane73,74 and by the small Tom5, Tom6, Tom7 proteins.75'77 The import of porin into the OMM needs the presence of at least Tom40, Tom5 and Tom7.76,77 The presence of a mitochondrial membrane potential is required for the translocation of most intramitochondrial precursors across the IMM.47 By contrast, the post-receptor import step leading to the insertion of a protein into the OMM does not require the presence of A\|/. The import of L-CPT I into the OMM is Av|/-independent51 (Fig. 1). The process by which a protein is inserted into the OMM is still a matter of debate. Current models for the mechanism of membrane insertion of a-helical transmembrane segments suggest that integration into the bilayer is coupled with the vectorial movement of the polypeptide through the import machinery. It is not known whether this movement arises from the release of the transmembrane segment laterally from the Tom complex or from the disassembly of the complex when it engages the transmembrane segment. For bitopic proteins, this occurs once during its translocation, whereas the process might be repeated for polytopic (multispanning) proteins. Thus, insertion of L-CPT I may be the result of threading back and forth across the OMM or, like the mechanism proposed for ß-barrel proteins, the result of simultaneous partitioning of the transmembrane segments as a bulk domain, in order to acquire a sufficient hydrophobic character to favour bilayer integration.

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