Human COQ10A and COQ10B are distinct lipid-binding START domain proteins required for coenzyme Q function.

Coenzyme Q (CoQ or ubiquinone) serves as an essential redox-active lipid in respiratory electron and proton transport during cellular energy metabolism. CoQ also functions as a membrane-localized antioxidant protecting cells against lipid peroxidation. CoQ deficiency is associated with multiple human diseases; CoQ10 supplementation in particular has noted cardioprotective benefits. In Saccharomyces cerevisiae, Coq10, a putative START domain protein, is believed to chaperone CoQ to sites where it functions. Yeast coq10 deletion mutants (coq10Δ) synthesize CoQ inefficiently during log phase growth and are respiratory defective and sensitive to oxidative stress. Humans have two orthologs of yeast COQ10, COQ10A and COQ10B Here, we tested the human co-orthologs for their ability to rescue the yeast mutant. We showed that expression of either human ortholog, COQ10A or COQ10B, rescues yeast coq10Δ mutant phenotypes, restoring the function of respiratory-dependent growth on a nonfermentable carbon source and sensitivity to oxidative stress induced by treatment with PUFAs. These effects indicate a strong functional conservation of Coq10 across different organisms. However, neither COQ10A nor COQ10B restored CoQ biosynthesis when expressed in the yeast coq10Δ mutant. The involvement of yeast Coq10 in CoQ biosynthesis may rely on its interactions with another protein, possibly Coq11, which is not found in humans. Coexpression analyses of yeast COQ10 and human COQ10A and COQ10B provide additional insights to functions of these START domain proteins and their potential roles in other biologic pathways.

Arm-site binding by lambda -integrase: solution structure and functional characterization of its amino-terminal domain.

The integrase protein (Int) from bacteriophage lambda catalyzes the insertion and excision of the viral genome into and out of Escherichia coli. It is a member of the lambda-Int family of site-specific recombinases that catalyze a diverse array of DNA rearrangements in archaebacteria, eubacteria, and yeast and belongs to the subset of this family that possesses two autonomous DNA-binding domains. The heterobivalent properties of Int can be decomposed into a carboxyl-terminal domain that executes the DNA cleavage and ligation reactions and a smaller amino-terminal domain that binds to an array of conserved DNA sites within the phage arms, thereby arranging Int protomers within the higher-order recombinogenic complex. We have determined that residues Met-1 to Leu-64 of Int constitute the minimal arm-type DNA-binding domain (INT-DBD(1-64)) and solved the solution structure by using NMR. We show that the INT-DBD(1-64) is a novel member of the growing family of three-stranded beta-sheet DNA-binding proteins, because it supplements this motif with a disordered amino-terminal basic tail that is important for arm-site binding. A model of the arm-DNA-binding domain recognizing its cognate DNA site is proposed on the basis of similarities with the analogous domain of Tn916 Int and is discussed in relation to other features of the protein.