Dumont MD, et al. (1990)

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.

PMID: 2154458
PubMed
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Reference Type: Journal Article | Research Support, Non-U.S. Gov't | Research Support, U.S. Gov't, P.H.S.
Authors: Dumont MD, Mathews AJ, Nall BT, Baim SB, Eustice DC, Sherman F
Primary Lit For: CYC1
Additional Lit For: CYC3 | CYC7
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