title: Differential stability of two apo-isocytochromes c in the yeast Saccharomyces cerevisiae.
abstract: The yeast Saccharomyces cerevisiae contains two forms of cytochrome c, iso-1-cytochrome c and iso-2-cytochrome c, encoded by the genes CYC1 and CYC7, respectively. The amino acid sequences of these two isozymes are approximately 80% identical. Cyc3- mutants lack both holocytochromes c, because of a deficiency of cytochrome c heme lyase, the enzyme catalyzing covalent attachment of the heme group to apocytochrome c. A deficiency of heme lyase also prevents import into mitochondria. Surprisingly, apo-iso-1-cytochrome c is absent in cyc3- strains, although apo-iso-2-cytochrome c is present at approximately the same level at which holo-iso-2-cytochrome c is found in CYC3+ strains. The lack of apo-iso-1-cytochrome c is not due to a deficiency of either transcription or translation, but to rapid degradation of the protein. Apocytochromes c encoded by composite cytochrome c genes composed of the central portion of iso-2-cytochrome c flanked by amino and carboxyl regions of iso-1-cytochrome c exhibit increased stability compared with apo-iso-1-cytochrome c. A region encompassing no more than four amino acid differences between iso-1- and iso-2-cytochromes c is sufficient to partially stabilize the protein. In contrast to what is observed in vivo with the apo forms, the holo forms of the composite isocytochromes c are even less stable to thermal denaturation than iso-1-cytochrome c or iso-2-cytochrome c. Either a small region of the sequence of apo-iso-1-cytochrome c is involved in degradation of the protein, or the corresponding region in apo-iso-2-cytochrome c is preventing degradation. The differential stability of the two isocytochromes c may be part of a regulatory process that increases the proportion of iso-2-cytochrome c under certain physiological conditions.
authors.affiliation: Department of Biochemistry, School of Medicine and Dentistry, Rochester, New York 14642.
authors.forename: M D
authors.initials: MD
authors.lastname: Dumont
authors.forename: A J
authors.initials: AJ
authors.lastname: Mathews
authors.forename: B T
authors.initials: BT
authors.lastname: Nall
authors.forename: S B
authors.initials: SB
authors.lastname: Baim
authors.forename: D C
authors.initials: DC
authors.lastname: Eustice
authors.forename: F
authors.initials: F
authors.lastname: Sherman
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D001051:Apoenzymes / Q000235:genetics*; D001059:Apoproteins / Q000235:genetics*; D001483:Base Sequence; D003574:Cytochrome c Group / Q000235:genetics*; D045304:Cytochromes c; D004271:DNA, Fungal / Q000235:genetics; D005800:Genes, Fungal; D005838:Genotype; D015151:Immunoblotting; D007527:Isoenzymes / Q000235:genetics*; D008969:Molecular Sequence Data; D011132:Polyribosomes / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D013816:Thermodynamics
chemical_list: D001051:Apoenzymes; D001059:Apoproteins; C014059:CYC1 protein, S cerevisiae; D003574:Cytochrome c Group; D004271:DNA, Fungal; D007527:Isoenzymes; D012333:RNA, Messenger; D029701:Saccharomyces cerevisiae Proteins; C032139:iso-2-cytochrome C; D045304:Cytochromes c