Differential Ubiquitin-dependent Degradation of the Yeast Apo-cytochrome cIsozymes*

The yeast Saccharomyces cerevisiaecontains two forms of cytochrome c, iso-1- and iso-2-cytochrome c, which are encoded by the nuclear genesCYC1and CYC7, respectively. The cytochromescare synthesized in the cytosol, imported into mitochondria, and subsequently modified by the covalent attachment of heme through the action of cytochrome cheme lyase, which is encoded by CYC3. Apo-iso-2-cytochrome cbut not apo-iso-1-cytochrome cwas observed incyc3−mutants. Furthermore, pulse-chase experiments previously demonstrated that the lack of apo-iso-1-cytochrome cwas due to its rapid degradation. We report herein that this degradation of apo-iso-1-cytochromecis dependent on ubiquitination and on the action of the proteasome. Diminished degradation of apo-iso-1-cytochromecwas observed in pre2-2and pre1-1mutants having altered proteasome subunits; in ubc1,ubc4, and ubc5strains lacking one or more of the ubiquitin-conjugating enzymes; and in strains blocked in multi-ubiquitination by overproduction of the abnormal ubiquitin-K48R ubiquitin. In addition, we have used epitope-tagged ubiquitin to demonstrate that apo-iso-1-cytochrome cbut not apo-iso-2-cytochrome cis ubiquitinated. Furthermore, the degradation of apo-iso-1-cytochrome cwas diminished when the N-terminal region was replaced with the N-terminal region of apo-iso-2-cytochrome c, indicating that this region may be the target for degradation. We suggest that ubiquitin-dependent degradation of apo-iso-1-cytochromecis part of the regulatory process controlling the preferential expression of the iso-cytochromes c.