The role of Cbp3p in cytochrome b expression and assembly into coenzyme QH2-cytochrome c reductase

Coenzyme QH2 -- cytochrome c reductase is a multi-subunit enzyme necessary for oxidative phosphorylation. CBP3 is a nuclear gene that encodes for a mitochondrial protein. Cells defective in Cbp3p expression are deficient in coenzyme QH2 -- cytochrome c reductase activity. Cbp3p is a membrane protein associated with the inner mitochondrial membrane. In S. cerevisiae carrying the deletion mutation for CBP3 the translation of cytochrome b is drastically reduced. The small amount of cytochrome b, relative to the wild type strain, that is translated is quickly degraded. However, after 25 minutes of translation in CBP3 point mutants, cytochrome b is translated well, relative to the wild type.;Although cytochrome b is translated in Cbp3p point mutants it is not present in steady-state, as determined through western blot analysis. In addition, cytochrome b is not detected in these point mutants by visible spectra, as in the strains carrying a null mutation for CBP3. Although, it appeared point mutants translate cytochrome b better than a CBP3 deletion strain, the stability and fate of that translation product is the same as in the null mutant, proteolytic degradation. Pulse-chase experiments have been used to demonstrate that rapid degradation of newly synthesized cytochrome b in the CBP3 point mutants is comparable to what is seen in null mutants. There is no difference in the mRNA level of cytochrome b in the null mutant as compared to the wild type. Cbp3p is also not involved in intron processing.;Finally, in CBP3 mutants newly translated cytochrome b is properly inserted into the inner mitochondrial membrane but not assembled into Coenzyme QH2 -- cytochrome c reductase. Cbp3p does not function in cytochrome b translation or insertion into the inner membrane of the mitochondria, it is however necessary for the assembly of this electron carrier into the coenzyme QH2 -- cytochrome c reductase complex.