Podcasts about mom72

Fri, 22 Apr 1994 12:00:00 +0100 https://epub.ub.uni-muenchen.de/7682/1/Neupert_Walter_7682.pdf Neupert, Walter; Pfanner, Nikolaus; Dietmeier, Klaus A.; Schlossmann, Jens

The novel genetic method of "sheltered RIP" (repeat induced point mutation) was used to generate a Neurospora crassa mutant in which MOM19, a component of the protein import machinery of the mitochondrial outer membrane, can be depleted. Deficiency in MOM19 resulted in a severe growth defect, but the cells remained viable. The number of mitochondrial profiles was not grossly changed, but mutant mitochondria were highly deficient in cristae membranes, cytochromes, and protein synthesis activity. Protein import into isolated mutant mitochondria was decreased by factors of 6 to 30 for most proteins from all suborganellar compartments. Proteins like the ADP/ATP carrier, MOM19, and cytochrome c, whose import into wild-type mitochondria occurs independently of MOM19 became imported normally showing that the reduced import activities are solely caused by a lack of MOM19. Depletion of MOM19 reveals a close functional relationship between MOM19 and MOM22, since loss of MOM19 led to decreased levels of MOM22 and reduced protein import through MOM22. Furthermore, MOM72 does not function as a general backup receptor for MOM19 suggesting that these two proteins have distinct precursor specificities. These findings demonstrate that the import receptor MOM19 fulfills an important role in the biogenesis of mitochondria and that it is essential for the formation of mitochondria competent in respiration and phosphorylation.

Mitochondrial protein import involves the recognition of preproteins by receptors and their subsequent translocation across the outer membrane. In Neurospora crassa, the two import receptors, MOM19 and MOM72, were found in a complex with the general insertion protein, GIP (formed by MOM7, MOM8, MOM30 and MOM38) and MOM22. We isolated a complex out of S. cerevisiae mitochondria consisting of MOM38/ISP42, the receptor MOM72, and five new yeast proteins, the putative equivalents of N. crassa MOM7, MOM8, MOM19, MOM22 and MOM30. A receptor complex isolated out of yeast cells transformed with N. crassa MOM19 contained the N. crassa master receptor in addition to the yeast proteins. This demonstrates that the yeast complex is functional, and provides strong evidence that we also have identified the yeast MOM19.

Mitochondria contain a complex machinery for the import of nuclear-encoded proteins. Receptor proteins exposed on the outer membrane surface are required for the specific binding of precursor proteins to mitochondria, either by binding of cytosolic signal recognition factors or by direct recognition of the precursor polypeptides. Subsequently, the precursors are inserted into the outer membrane at the general insertion site GIP (general insertion protein. Here we report the analysis of receptors and GIP by crosslinking of translocation intermediates and by coimmunoprecipitation. Surface-accumulated precursors were cross-linked to the receptors MOM19 and MOM72, suggesting a direct interaction of preproteins with surface receptors. We identified three novel mitochondrial outer membrane proteins, MOM7, MOMS, and MOM30 that, together with the previously identified MOM38, seem to form the GIP site and are present in the mitochondrial receptor complex.

The specific targeting of precursor proteins synthesized in the cytosol to various cell organelles is a central aspect of intracellular protein traffic. Several hundred different proteins are imported from the cytosol into the mitochondria. Recent studies have identified the mitochondrial outer membrane proteins MOM19, MOM72, MOM38 (≈ISP42) and p32 which have a role in initial steps of protein import. The first three components are present in a multi-subunit complex that catalyses recognition and membrane insertion of precursor proteins.

We have identified a mitochondrial outer membrane protein of 72 kd (MOM72) that exhibits the properties of an import receptor for the ADP/ATP carrier (AAC), the most abundant mitochondrial protein. Monospecific antibodies and Fab fragments against MOM72 selectively inhibit import of AAC at the level of specific binding to the mitochondria. AAC bound to the mitochondrial surface is coprecipitated with antibodies against MOM72 after lysis of mitochondria with detergent. MOM72 thus has a complementary function to that of MOM19, which acts as an import receptor for the majority of mitochondrial proteins studied so far but not for the AAC. The import pathway of the precursor of MOM72 appears to involve MOM19 as receptor.

We have identified the yeast homologue of Neurospora crassa MOM72, the mitochondrial import receptor for the ADP/ATP carrier (AAC), by functional studies and by cDNA sequencing. Mitochondria of a yeast mutant in which the gene for MOM72 was disrupted were impaired in specific binding and import of AAC. Unexpectedly, we found a residual, yet significant import of AAC into mitochondria lacking MOM72 that occurred via the receptor MOM19. We conclude that both MOM72 and MOM19 can direct AAC into mitochondria, albeit with different efficiency. Moreover, the precursor of MOM72 apparently does not require a positively charged sequence at the extreme amino terminus for targeting to mitochondria.