Interactions among three proteins that specifically activate translation of the mitochondrial COX3 mRNA in Saccharomyces cerevisiae.

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Abstract
The PET54, PET122, and PET494 proteins, which are associated with the yeast inner mitochondrial membrane, specifically activate translation of the mitochondrially encoded COX3 mRNA. We used the two-hybrid system to test whether pairs of these proteins, when fused to either the GAL4 DNA-binding or transcriptional activating domain, can physically associate as measured by the expression of the GAL4-dependent reporter, lacZ. PET54 and PET122 interacted in this system, and an amino-terminally truncated PET494 fragment showed an interaction with PET54. We also detected functional interactions between PET54 and PET122 genetically: a pet54 missense substitution (Phe to Gly at position 244) that caused a severe respiratory defect was suppressed both by a missense substitution affecting PET122 (Gly to Val at position 211) and by overproduction of wild-type PET122. Both Gly and Ala, substituted at PET54 position 244, disrupted the two-hybrid interactions with PET122 and PET494. While Ala at PET54 position 244 caused only a modest respiratory phenotype alone, it caused a severe respiratory defect when combined with a cold-sensitive mitochondrial mutation affecting the COX3 mRNA 5' leader. This synthetic defect was suppressed by a missense substitution in PET122 and by overproduction of wild-type PET122, indicating functional interactions among PET54, PET122, and the mRNA. Taken together with previous work, these data suggest that a complex containing PET54, PET122, and PET494 mediates the interaction of the COX3 mRNA with mitochondrial ribosomes at the surface of the inner membrane.