Precursors of mitochondrial proteins are degraded in the cytosol at different rates

Alterations in coenzyme Q10 status in a cybrid line harboring the 3243A>G mutation of mitochondrial DNA is associated with abnormal mitochondrial bioenergetics and dysregulated mitochondrial biogenesis

Mitochondrial DNA (mtDNA) mutations, including the m.3243A>G mutation that causes mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), are associated with secondary coenzyme Q10 (CoQ10) deficiency. We previously demonstrated that PPARGC1A knockdown repressed the expression of PDSS2 and several COQ genes. In the present study, we compared the mitochondrial function, CoQ10 status, and levels of PDSS and COQ proteins and genes between mutant cybrids harboring the m.3243A>G mutation and wild-type cybrids. Decreased mitochondrial energy production, defective respiratory function, and reduced CoQ10 levels were observed in the mutant cybrids. The ubiquinol-10:ubiquinone-10 ratio was lower in the mutant cybrids, indicating blockage of the electron transfer upstream of CoQ, as evident from the reduced ratio upon rotenone treatment and increased ratio upon antimycin A treatment in 143B cells. The mutant cybrids exhibited downregulation of PDSS2 and several COQ genes and upregulation of COQ8A. In these cybrids, the levels of PDSS2, COQ3-a isoform, COQ4, and COQ9 were reduced, whereas those of COQ3-b and COQ8A were elevated. The mutant cybrids had repressed PPARGC1A expression, elevated ATP5A levels, and reduced levels of mtDNA-encoded proteins, nuclear DNA-encoded subunits of respiratory enzyme complexes, MNRR1, cytochrome c, and DHODH, but no change in TFAM, TOM20, and VDAC1 levels. Alterations in the CoQ10 level in MELAS may be associated with mitochondrial energy deficiency and abnormal gene regulation. The finding of a reduction in the ubiquinol-10:ubiquinone-10 ratio in the MELAS mutant cybrids differs from our previous discovery that cybrids harboring the m.8344A>G mutation exhibit a high ubiquinol-10:ubiquinone-10 ratio.

Regulation of enzyme levels by proteolysis: the role of pest regions

Enzymes can be regulated in a variety of ways. Readily reversible mechanisms, such as phosphorylation, are frequently used by cells to control metabolic pathways. Less often, enzyme levels are regulated by changing the rate at which the protein is destroyed. Although these changes, too, are reversible through protein synthesis, large variations in enzyme concentration can be produced in very short periods of time by combinations of transcriptional control, translational control and rapid degradation. We recently examined the primary sequences of proteins whose intracellular half-lives are less than two hours. With a single exception, each short-lived protein contains one or more regions rich in proline (P), glutamic acid (E), serine (S) and threonine (T). These PEST regions range in length from 12 to 60 residues, and they are often flanked by possibly charged amino acids. Similar inspection of 35 more stable, structurally characterized proteins revealed only three weak PEST regions. All PEST proteins appear to be important regulatory molecules, and their fast turnover surely reflects a metabolic requirement for rapid changes in their concentrations. Known PEST proteins include oncogene products, key enzymes and components of signal pathways. In addition, there are a number of PEST-containing proteins that are suspected of being rapidly degraded. These proteins include Drosophila homeotic proteins (e.g., notch, snake, caudal, ftz and even-skipped) and a host of yeast cdc mutants. PEST regions, which target the molecules containing them for destruction, thus appear to be widely distributed among metabolically unstable proteins.