Cargo sequences are important for Som1p-dependent signal peptide cleavage in yeast mitochondria

Set1-mediated H3K4 methylation is required for Candida albicans virulence by regulating intracellular level of reactive oxygen species

Candida albicans is an opportunistic human fungal pathogen that exists in normal flora but can cause infection in immunocompromised individuals. The transition to pathogenic C. albicans requires a change of various gene expressions. Because histone-modifying enzymes can regulate gene expression, they are thought to control the virulence of C. albicans. Indeed, the absence of H3 lysine 4 (H3K4) methyltransferase Set1 has been shown to reduce the virulence of C. albicans; however, Set1-regulated genes responsible for this attenuated virulence phenotype remain unknown. Here, we demonstrated that Set1 positively regulates the expression of mitochondrial protein genes by methylating H3K4. In particular, levels of cellular mitochondrial reactive oxygen species (ROS) were higher in Δset1 than in the wild-type due to the defect of those genes’ expression. Set1 deletion also increases H₂O₂ sensitivity and prevents proper colony formation when interacting with macrophage in vitro, consistent with its attenuated virulence in vivo. Together, these findings suggest that Set1 is required to regulate proper cellular ROS production by positively regulating the expression of mitochondrial protein genes and subsequently sustaining mitochondrial membrane integrity. Consequently, C. albicans maintains proper ROS levels via Set1-mediated transcriptional regulation, thus establishing a rapid defense against external ROS generated by the host.

Mitochondrial disulfide relay: redox-regulated protein import into the intermembrane space

99% of all mitochondrial proteins are synthesized in the cytosol, from where they are imported into mitochondria. In contrast to matrix proteins, many proteins of the intermembrane space (IMS) lack presequences and are imported in an oxidation-driven reaction by the mitochondrial disulfide relay. Incoming polypeptides are recognized and oxidized by the IMS-located receptor Mia40. Reoxidation of Mia40 is facilitated by the sulfhydryl oxidase Erv1 and the respiratory chain. Although structurally unrelated, the mitochondrial disulfide relay functionally resembles the Dsb (disufide bond) system of the bacterial periplasm, the compartment from which the IMS was derived 2 billion years ago.

Genome-wide analysis of eukaryotic twin CX9C proteins

Twin CX(9)C proteins are eukaryotic proteins that derive their name from their characteristic motif, consisting of two pairs of cysteines that form two disulfide bonds stabilizing a coiled coil-helix-coiled coil-helix (CHCH) fold. The best characterized of these proteins are Cox17, a copper chaperone acting in cytochrome c oxidase biogenesis, and Mia40, the central component of a system for protein import into the mitochondrial inter-membrane space (IMS). However, the range of possible functions for these proteins is unclear. Here, we performed a systematic search of twin CX(9)C proteins in eukaryotic organisms, and classified them into groups of putative homologues, by combining bioinformatics methods with literature analysis. Our results suggest that the functions of most twin CX(9)C proteins vary around the common theme of playing a scaffolding role, which can tie their observed roles in mitochondrial structure and function. This study will enhance the present annotation of eukaryotic proteomes, and will provide a rational basis for future experimental work aimed at a deeper understanding of this remarkable class of proteins.

A yeast assay probes the interaction between botulinum neurotoxin serotype B and its SNARE substrate

The seven functionally distinct serotypes (A-G) of botulinum neurotoxin (BoNT) are dichains consisting of light chain (LC) with zinc-dependent endoprotease activity connected by one disulfide bond to heavy chain with neuronal-cell translocation and receptor-binding domains. LC-mediated proteolysis of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and consequent inhibition of synaptic vesicle fusion to the presynaptic membrane of human motor neurons are responsible for flaccid paralysis associated with botulism. LC endoproteolysis is complex, requiring highly extended SNARE sequences at the surface of intracellular membranes and prompting our development of a genetically amenable assay to monitor the interaction between BoNT/LC and its SNARE substrate. Using BoNT serotype B as a model, the assay employs a chimeric SNARE protein where a portion of neuronal synaptobrevin (Sb) is fused to Snc2p, a Sb ortholog required for protein secretion from yeast cells. Regulated expression of serotype B-LC in yeast leads to cleavage of the chimera and a conditional growth defect. To assess utility of this assay for monitoring SNARE protein cleavage, we growth-selected chimeric SNARE mutations that inhibited proteolysis. When these mutations were introduced into Sb and examined for cleavage, substrate residues located near and distal to the cleavage site were important, including residues positioned near the Sb transmembrane domain, an unexplored aspect of BoNT cell intoxication. Additional mutations were positioned in a nine-residue SNARE motif, supporting a previously assigned role for this motif in LC recognition and providing proof of principle for the application of yeast-based technology to study intracellular BoNT/LC endoproteases.

Substrate specificity of inner membrane peptidase in yeast mitochondria

The inner membrane protease (IMP) cleaves intra-organelle sorting peptides from precursor proteins in mitochondria of the yeast Saccharomyces cerevisiae. An unusual feature of the IMP is the presence of two catalytic subunits, Imp1p and Imp2p, which recognize distinct substrate sets even though both enzymes belong to the same protease family. This nonoverlapping substrate specificity was hypothesized to result from the recognition of distinct residues at the P'1 position (also termed +1 position) in the protease substrates. Here, we constructed an extensive series of mutations to obtain a profile of the critical cleavage site residues in IMP substrates and conclude that Imp1p, and not Imp2p, recognizes specific P'1 residues. In addition to its specificity for P'1 residues, Imp1p also shows substrate specificity for the P3 (-3) position. In contrast, Imp2p recognizes the P1 (-1) position and the P3 position. Based on this new understanding of IMP substrate specificity, we conducted a survey for candidate IMP substrates in mammalian mitochondria and found consensus Imp2p cleavage sites in mammalian precursors to cytochrome c1 and glycerol-3-phosphate (G-3-P) dehydrogenase. Presence of a putative Imp2p cleavage site in G-3-P dehydrogenase was surprising, as its yeast ortholog contains an Imp1p cleavage site. To address this issue experimentally, we performed the first co-expression of mammalian IMP with proposed mammalian IMP precursors in yeast and show that murine precursors to cytochrome c1 and G-3-P dehydrogenase are cleaved by murine Imp2p. These results suggest, surprisingly, G-3-P dehydrogenase has switched from Imp1p in yeast to Imp2p in mammals.