Cox25 Teams Up with Mss51, Ssc1, and Cox14 to Regulate Mitochondrial Cytochrome c Oxidase Subunit 1 Expression and Assembly in Saccharomyces cerevisiae

Abstract

In the yeast Saccharomyces cerevisiae, mitochondrial cytochrome c oxidase (COX) biogenesis is translationally regulated. Mss51, a specific COX1 mRNA translational activator and Cox1 chaperone, drives the regulatory mechanism. During translation and post-translationally, newly synthesized Cox1 physically interacts with a complex of proteins involving Ssc1, Mss51, and Cox14, which eventually hand over Cox1 to the assembly pathway. This step is probably catalyzed by assembly chaperones such as Shy1 in a process coupled to the release of Ssc1-Mss51 from the complex. Impaired COX assembly results in the trapping of Mss51 in the complex, thus limiting its availability for COX1 mRNA translation. An exception is a null mutation in COX14 that does not affect Cox1 synthesis because the Mss51 trapping complexes become unstable, and Mss51 is readily available for translation. Here we present evidence showing that Cox25 is a new essential COX assembly factor that plays some roles similar to Cox14. A null mutation in COX25 by itself or in combination with other COX mutations does not affect Cox1 synthesis. Cox25 is an inner mitochondrial membrane intrinsic protein with a hydrophilic C terminus protruding into the matrix. Cox25 is an essential component of the complexes containing newly synthesized Cox1, Ssc1, Mss51, and Cox14. In addition, Cox25 is also found to interact with Shy1 and Cox5 in a complex that does not contain Mss51. These results suggest that once Ssc1-Mss51 are released from the Cox1 stabilization complex, Cox25 continues to interact with Cox14 and Cox1 to facilitate the formation of multisubunit COX assembly intermediates.

hCOA3 Stabilizes Cytochrome c Oxidase 1 (COX1) and Promotes Cytochrome c Oxidase Assembly in Human Mitochondria

“…We have observed hCOA3-FLAG is forming several high molecular weight complexes, which could constitute COX subassemblies as well as CIII-CIV supercomplexes, as has already been observed for its yeast homologue Coa3 (37,38). The physical interaction with COX subunits was demonstrated by immunoprecipitation studies followed by mass spectrometric and immunoblot analyses.…”

“…Mss51, the central protein of a negative feedback translational regulatory system in yeast, has a dual function acting as a translational activator of COX1 mRNA and as a COX1 chaperone during the first steps of COX biogenesis (39). During and after synthesis, COX1 is bound to Mss51 in a complex stabilized by two small COX-specific chaperones, Cox14 (40) and Coa3 (37,38) and the mitochondrial Hsp70 chaperone (41). When COX1 pro-FIGURE 8. hCOA3 interacts with COX structural subunits and complex I.…”

Mechanisms of mitochondrial translational regulation

“…domains of Cox1 and direct their insertion into the inner membrane. Definitively, they are required for the stability of the Mss51 preassembly complex as in cox14 and coa3 mutants all Mss51 is engaged with Ssc1 in the heterodimer and it is available to promote normal levels of Cox1 synthesis (8,28,29). However, this newly synthesized Cox1 fails to be assembled and is rapidly degraded, suggesting additional roles of Cox14 and Coa3 in COX biogenesis (8,28,29).…”

“…Mss51 dynamically interacts with newly synthesized Cox1, promoting its stability and its incorporation into COX subcomplexes, and with several components of the COX biogenetic process (26,27). During Cox1 synthesis on the mitochondrial ribosomes, Mss51 interacts with newly synthesized Cox1, in a complex that also contains the mitochondrial Hsp70 chaperone Ssc1, its cochaperone Mdj1 and the two COX assembly factors Cox14 and Coa3 (26,28). After Cox1 synthesis and Oxa1-dependent membrane insertion, an Ssc1-Mss51-Cox1-Cox14-Coa3 preassembly complex is formed (26,28,29).…”

“…During Cox1 synthesis on the mitochondrial ribosomes, Mss51 interacts with newly synthesized Cox1, in a complex that also contains the mitochondrial Hsp70 chaperone Ssc1, its cochaperone Mdj1 and the two COX assembly factors Cox14 and Coa3 (26,28). After Cox1 synthesis and Oxa1-dependent membrane insertion, an Ssc1-Mss51-Cox1-Cox14-Coa3 preassembly complex is formed (26,28,29). This complex, abundant in wild-type cells, has been proposed to represent a reservoir of stable Cox1 ready to be matured and/or progress in the COX assembly process.…”