Saccharomyces cerevisiae iso-1-cytochrome c was conjugated with ubiquitin (Ub) in vitro in a rabbit reticulocyte extract (Fraction II). By N-terminal protein sequencing, it was found for both the mono- and diubiquitinated products that the major Ub attachment site is on Lys4 (residue 9) of the cytochrome c. Thus, the residue ubiquitinated in iso-1-cytochrome c is identical with that previously determined for the yeast iso-2 form (Sokolik, C. W., and Cohen, R. E. (1991) J. Biol. Chem. 266, 9100-9107). For both cytochromes c, the proportions of diubiquitinated and higher order conjugates are drastically reduced when Ub is replaced with a Lys48----Arg variant, suggesting that the Ub-Ub moieties are linked predominantly through Lys48. Despite close similarities in structure and ubiquitination sites, conjugation to iso-2-cytochrome c is approximately 5-fold faster than for the iso-1 form; vertebrate cytochromes c are even poorer substrates, being ubiquitinated at only approximately 5% of the rate of the iso-2 protein. Comparison of several cytochrome c variants excludes alpha-N-acetylation or the identity of the N-terminal amino acid as the important recognition determinants in these reactions. The results, which include the finding that ferro and ferri-iso-2-cytochromes c are ubiquitinated equally, also are evidence against a simple correlation between ubiquitination efficiency and thermodynamic stability. Rather, the presence of a pair of lysines (Lys4-Lys5) within the relatively unstructured N-terminal extension of the yeast cytochromes c may be responsible for their preferential ubiquitination.

• PMID: 1309759
• PubMed
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Reference Type

Journal Article | Research Support, U.S. Gov't, P.H.S.

Authors

Sokolik CW, Cohen RE

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