"However, all proteins of the mitochondrial outer membrane and some proteins of the intermembrane space (IMS) and the inner membrane are devoid of such signals. Among the latter proteins, the family of solute carrier proteins together with some Tim components comprise a subclass of inner membrane proteins that uses a unique pathway for mitochondrial import (Koehler et al., 1999b; Tokatlidis and Schatz, 1999). After their synthesis on cytosolic ribosomes, these precursor proteins are protected against aggregation by interaction with molecular chaperones which also help to direct them to the import receptor Tom70 (Komiya et al., 1997; Young et al., 2003). "
[Show abstract][Hide abstract] ABSTRACT: Precursor proteins of the solute carrier family and of channel forming Tim components are imported into mitochondria in two main steps. First, they are translocated through the TOM complex in the outer membrane, a process assisted by the Tim9/Tim10 complex. They are passed on to the TIM22 complex, which facilitates their insertion into the inner membrane. In the present study, we have analyzed the function of the Tim9/Tim10 complex in the translocation of substrates across the outer membrane of mitochondria. The purified TOM core complex was reconstituted into lipid vesicles in which purified Tim9/Tim10 complex was entrapped. The precursor of the ADP/ATP carrier (AAC) was found to be translocated across the membrane of such lipid vesicles. Thus, these components are sufficient for translocation of AAC precursor across the outer membrane. Peptide libraries covering various substrate proteins were used to identify segments that are bound by Tim9/Tim10 complex upon translocation through the TOM complex. The patterns of binding sites on the substrate proteins suggest a mechanism by which portions of membrane-spanning segments together with flanking hydrophilic segments are recognized and bound by the Tim9/Tim10 complex as they emerge from the TOM complex into the intermembrane space.
Molecular Biology of the Cell 04/2004; 15(3):1445-58. DOI:10.1091/mbc.E03-05-0272 · 4.55 Impact Factor
"The first clue that YidC could possibly be involved in the membrane insertion of bacterial proteins came from studies in the mitochondrial field investigating the membrane protein insertion into the mitochondrial inner membrane. The conservative pathway inserts proteins into the mitochondrial inner membrane from the matrix compartment in a manner dependent on the Oxa1p protein (Dalbey and Kuhn, 2000; Tokatlidis and Schatz, 1999). Proteins requiring Oxa1p for insertion are found to be either nuclear encoded (Hell et al., 1997; Hell et al., 1998) or mitochondrial encoded (Hell et al., 2001). "
[Show abstract][Hide abstract] ABSTRACT: Abstract: The Sec complex is the major translocase to mediate membrane protein insertion in bacteria, but there exists some proteins whose insertion is independent of the Sec complex. In year 2000, YidC was discovered that is essential for insertion of proteins into membranes in bacteria, especially for the Sec-independent proteins. Chapter 1 is an in-depth review about YidC in membrane protein assembly. In Chapter 2, we found YidC interacts with leader peptidase during its membrane insertion using crosslinking techniques. These data combined with the in vivo depletion study of YidC (James Samuelson₂s dissertation, 2000) provide strong evidence to support YidC is a translocase component in bacteria. In Chapter 3, I report that the Sec-independent Pf3 coat protein requires YidC for insertion into the membrane. Using photocrosslinking techniques, we find that Pf3 coat interacts strongly with YidC only after its transmembrane segment is fully exposed outside the ribosome tunnel. Interaction between Pf3 coat and YidC occurs even when the proton motive force does not function on Pf3 coat. Our study demonstrates that YidC can directly interact with a Sec-independent membrane protein and its role is to fold the Pf3 protein into a transmembrane configuration. In Chapter 4, we found incorporation of site-specific protease sites into YidC results in YidC temperature-sensitive (ts) or cold-sensitive (cs) mutants. The YidC ts and cs strains were then constructed. The membrane insertion of the Sec-independent M13 procoat protein is inhibited in YidC ts strains when the cells were grown at 42 oC for 20 minutes. This provides the strongest evidence thus far that YidC plays a direct role in membrane protein insertion. Using the cs YidC strain, we find the insertion of the Sec-dependent leader peptidase is inhibited at 25 oC, whereas the insertion of M13 procoat is nearly normal. The cs YidC mutant shows a reduced interaction with the Sec machinery. These data suggest that the cold-sensitive YidC mutant is blocked in the Sec-related function, while its activity for inserting procoat is functioning almost normally. These properties of the cold-sensitive mutant strongly support the idea that YidC can function alone or with the Sec machinery. Title from first page of PDF file. Document formatted into pages; contains xviii, 193 p.: ill. (some col.). Includes abstract and vita. Advisor: Ross E. Dalbey, Biochemistry Program. Thesis (Ph. D.)--Ohio State University, 2002. Includes bibliographical references (p. 130-139). System requirements: World Wide Web browser and PDF viewer.
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