SCO1 and SCO2 Act as High Copy Suppressors of a Mitochondrial Copper Recruitment Defect in Saccharomyces cerevisiae

Abstract

C129/U1 is a respiratory defective mutant of Saccharomyces cerevisiae arrested in cytochrome oxidase assembly due to a mutation in COX17, a nuclear gene encoding a low molecular weight cytoplasmic protein proposed to function in mitochondrial copper recruitment. In the present study we show that the respiratory defect of C129/U1 is rescuable by two multicopy suppressors, SCO1 and SCO2. SCO1 was earlier reported to code for a mitochondrial inner membrane protein with an essential function in cytochrome oxidase assembly (Buchwald, P., Krummeck, G., and Rodel, G. (1991) Mol. Gen. Genet. 229, 413-420). SCO2 is a homologue of SCO1, whose product is also localized in the mitochondrial membrane but is not required for respiration.SCO1 also suppresses a cox17 null mutant, indicating that overexpression of Sco1p can compensate for the absence of Cox17p. In contrast, neither copper, COX17 on a multicopy plasmid, or a combination of the two is able to restore respiration in sco1 mutants. Rescue of cox17 mutants by Sco1p suggests that this mitochondrial protein plays a role either in mitochondrial copper transport or insertion of copper into the active site of cytochrome oxidase. Although SCO2 can also partially restore respiratory growth in the cox17 null mutant, rescue in this case requires addition of copper to the growth medium. SCO2 does not suppress a sco1 null mutant, although it is able to partially rescue a sco1 point mutant. We interpret the ability of SCO2 to restore respiration in cox17, but not in sco1 mutants, to indicate that Sco1p and Sco2p have overlapping but not identical functions.

Molecular characterization of Saccharomyces cerevisiae Sco2p reveals a high degree of redundancy with Sco1p

“…Overexpression of SCO2 is not capable to suppress the respiratory deficiency of sco1 null mutants (Glerum et al, 1996). However, as we have demonstrated previously, the C-terminal half of Sco1p can be replaced by the corresponding part of Sco2p, suggesting a partially overlapping function of Sco1p and Sco2p in yeast .…”

“…This observation suggests that Sco2p, too, might be involved in mt copper metabolism. Interestingly, however, the disruption of SCO2 does not result in a respiratory deficient phenotype (Glerum et al, 1996).…”

“…Contrary to the human counterparts which are both essential for mt function, the role of Sco2p in yeast is enigmatic: like in the case of Sco1p, overexpression of Sco2p can suppress the lack of the mt copper shuttle protein Cox17p, albeit only in the presence of elevated copper concentrations (Glerum et al, 1996). This observation suggests that Sco2p, too, might be involved in mt copper metabolism.…”

“…Both cysteine residues of the CXXXC-motif, C 148 and C 152 , as well as H 239 have been shown to be essential for Sco1p function Lode et al, 2000). The first hint at a role of Sco1p in mt copper metabolism came from the observation that the respiratory deficiency of yeast cells lacking the mt copper carrier Cox17p can be suppressed by overexpression of Sco1p (Glerum et al, 1996). Sco1p possesses characteristic features of a copper chaperon: recent data show that one Sco1p molecule binds one Cu 1+ ion via the essential amino acids C 148 , C 152 and H 239 (Nittis et al, 2001).…”

“…Sco2p shares a number of common features with Sco1p: it is a mt membrane protein of about 30 kDa. Its N-terminal sequence of about 4 kDa, reminiscent of mt targeting sequences, is probably cleaved off during import by processing of the precursor protein (Glerum et al, 1996). Anchoring in the membrane is likely to be mediated by a single hydrophobic stretch, which is predicted as a TM segment.…”