Escherichia coli

Trigger factor is a molecular chaperone that is present in all species of eubacteria. It binds to the ribosomal 50S subunit near the translation exit tunnel and is thought to be the first protein to interact with nascent polypeptides emerging from the ribosome. The chaperone has a peptidyl-prolyl cis-trans isomerase (PPIase) activity that catalyzes the rate-limiting proline isomerization in the protein-folding process. We have determined the crystal structure of nearly full-length trigger factor from Vibrio cholerae by x-ray crystallography at 2.5-Å resolution. The structure is composed of two trigger-factor molecules related by a noncrystallographic twofold symmetry axis. The monomer has an elongated shape and is folded into three domains: an N-terminal domain I that binds to the ribosome, a central domain II that contains PPIase activity, and a C-terminal domain III. The active site of the PPIase domain is occupied by a loop from domain III, suggesting that the PPIase activity of the protein could be regulated. The dimer interface is formed between domains I and III and contains residues of mixed properties. Further implications about dimerization, ribosome binding, and other functions of trigger factor are discussed.

Mitochondria are the main site for generating reactive oxygen species, which are key players in diverse biological processes. However, the molecular pathways of redox signal transduction from the matrix to the cytosol are poorly defined. Here we report an inside-out redox signal of mitochondria. Cysteine oxidation of MIC60, an inner mitochondrial membrane protein, triggers the formation of disulfide bonds and the physical association of MIC60 with Miro, an outer mitochondrial membrane protein. The oxidative structural change of this membrane-crossing complex ultimately elicits cellular responses that delay mitophagy, impair cellular respiration, and cause oxidative stress. Blocking the MIC60-Miro interaction or reducing either protein, genetically or pharmacologically, extends lifespan and health-span of healthy fruit flies, and benefits multiple models of Parkinson's disease and Friedreich's Ataxia. Our discovery provides a molecular basis for common treatment strategies against oxidative stress.

The orchestration of mitochondria within the cell represents a critical aspect of cell biology. At the center of this process is the outer mitochondrial membrane protein, Miro. Miro coordinates diverse cellular processes by regulating connections between organelles and the cytoskeleton that range from mediating contacts between the endoplasmic reticulum and mitochondria to the regulation of both actin and microtubule motor proteins. Recently, a number of cell biological, biochemical, and protein structure studies have helped to characterize the myriad roles played by Miro. In addition to answering questions regarding Miro's function, these studies have opened the door to new avenues in the study of Miro in the cell. This review will focus on summarizing recent findings for Miro's structure, function, and activity while highlighting key questions that remain unanswered.

Mitochondrial F1-ATPase contains a hexamer of alternating α and β subunits. The assembly of this structure requires two specialized chaperones, Atp11p and Atp12p, that bind transiently to β and α. In the absence of Atp11p and Atp12p, the hexamer is not formed, and α and β precipitate as large insoluble aggregates. An early model for the mechanism of chaperone-mediated F1 assembly (Wang, Z. G., Sheluho, D., Gatti, D. L., and Ackerman, S. H. (2000) EMBO J. 19, 1486–1493) hypothesized that the chaperones themselves look very much like the α and β subunits, and proposed an exchange of Atp11p for α and of Atp12p for β; the driving force for the exchange was expected to be a higher affinity of α and β for each other than for the respective chaperone partners. One important feature of this model was the prediction that as long as Atp11p is bound to β and Atp12p is bound to α, the two F1 subunits cannot interact at either the catalytic site or the noncatalytic site interface. Here we present the structures of Atp11p from Candida glabrata and Atp12p from Paracoccus denitrificans, and we show that some features of the Wang model are correct, namely that binding of the chaperones to α and β prevents further interactions between these F1 subunits. However, Atp11p and Atp12p do not resemble α or β, and it is instead the F1 γ subunit that initiates the release of the chaperones from α and β and their further assembly into the mature complex.