Altered iso-1- and iso-2-cytochromes c, with certain amino acid replacements, occur at diminished levels due to degradation in the yeast Saccharomyces cerevisiae. A subclass of the labile isocytochromes c are significantly protected from degradation by the presence of cytochromes a.a3 and c1, the physiological partners of cytochrome c. We have investigated the degradation that is dependent on physiological partners by examining the levels of iso-1-cytochrome c having all or most amino acid replacements at positions 6, 41, 52, and 78, in both rho+ strains and rho- strains, which lacks cytochrome a.a3. In addition, we have examined some of these replacements in strains also having the N52I replacement, which suppresses a variety of abnormal iso-1-cytochromes c, including those whose degradation is either dependent or independent on the physiological partners. Although some degree of preferential rho--dependent reductions was observed for iso-1-cytochromes c having replacements at each of the 6, 41, 52, and 78 sites, prominent effects of rho+/rho- ratios of approximately 100/0 to 30/0 were observed for iso-1-cytochromes c having replacements mainly at the 41, 52, and 78 sites, but not the G6 site. We suggest that prominent degradation dependent on physiological partners may be restricted to certain regions of the cytochrome c molecule. Furthermore, we suggest that the region of the largest confirmational difference between oxidized and reduced cytochrome c appears to be particularly protected by interactions with its physiological partners.
Copyright 1998 Academic Press.