Figure 11.

A model for Coq10/START domain polypeptide function in de novo Q₆ biosynthesis and in delivery of Q₆ to respiratory chain complexes. (A) In wild-type cells, the Coq10:Q protein:ligand complex is postulated to deliver Q₆ to the multisubunit Coq polypeptide complex and so enhance stability of the Coq polypeptides and de novo synthesis of Q₆. Newly synthesized Q₆ in the mitochondrial inner membrane is delivered to respiratory chain complexes and can function as an antioxidant. We postulate that Coq10:Q₆ may also deliver Q₆ to respiratory complexes. (B) The coq10 null mutant contains lower steady state levels of Coq polypeptides [20] and is shown with a less stable multisubunit Coq complex. Impaired de novo synthesis of Q₆ and lack of Coq10:Q₆ delivery to respiratory complexes accounts for the inefficient respiration observed in the coq10 null mutant. (C) The coq10 null mutant can be rescued by over-expression of COQ8, via its ability to restore the Coq polypeptide complex [25]. We postulate that the enhanced de novo Q₆ biosynthesis formed by the Coq multisubunit complex is efficiently delivered to respiratory complexes, despite the absence of Coq10p.