Coupling chemical energy by the hsp70/tim44 complex to drive protein translocation into mitochondria

The triage of damaged proteins: degradation by the ubiquitin-proteasome pathway or repair by molecular chaperones

Accumulation of damaged proteins is causally related to many age-related diseases. The ubiquitin-proteasome pathway (UPP) plays a role in selective degradation of damaged proteins, whereas molecular chaperones, such as heat shock proteins, are involved in refolding denatured proteins. This work demonstrates for the first time that the UPP and molecular chaperones work in a competitive manner and that the fates of denatured proteins are determined by the relative activities of the UPP and molecular chaperones. Enhanced UPP activity suppresses the refolding of denatured proteins whereas elevated chaperone activity inhibits the degradation of denatured proteins. CHIP, a co-chaperone with E3 activity, plays a pivotal role in determining the fates of the damaged proteins. The delicate balance between UPP-mediated degradation and refolding of denatured proteins is governed by relative levels of CHIP and other molecular chaperones. Isopeptidases, the enzymes that reverse the actions of CHIP, also play an important role in determining the fate of denatured proteins.

Functional reconstitution of the import of the yeast ADP/ATP carrier mediated by the TIM10 complex

Import of the ADP/ATP carrier (AAC) into mitochondria requires the soluble TIM10 complex to cross the intermembrane space. We report here that Tim9 and Tim10 purified from Escherichia coli can form a complex of the same size as the endogenous complex from yeast mitochondria. This shows that no other mitochondrial protein is required for the formation of the TIM10 complex. Co-expression of both proteins rendered Tim9 more soluble and allowed purification of the reconstituted complex in a single step. Urea/EDTA treatment of recombinant Tim10 allowed its import into tim10-ts mitochondria that lack endogenous Tim10 and cannot import AAC. In this way, we were able to (i) reconstitute the TIM10 complex in the intermembrane space and (ii) restore import of AAC to almost wild-type levels. The reconstituted TIM10 complex not only facilitated passage of AAC across the outer membrane but also ensured its accurate membrane insertion. We conclude that the TIM10 complex can be formed exclusively from Tim9 and Tim10 and that the reconstituted complex efficiently restores AAC import in a strain lacking the TIM10 complex.

Identification and characterization of molecular interactions between glucose-regulated proteins (GRPs) mortalin/GRP75/peptide-binding protein 74 (PBP74) and GRP94

A heat-shock protein (hsp) 70 family member mortalin/glucose-regulated protein (GRP) 75/peptide-binding protein 74 (PBP74) has been localized to various cellular compartments including mitochondria, endoplasmic reticulum and cytoplasmic vesicles. Here we describe its interactions with an endoplasmic reticulum protein GRP94, a member of the hsp90 family of GRPs. Interactions were identified, confirmed and characterized by far-Western screening, in vivo reporter and co-immunoprecipitation assays. Interacting domains of the two proteins were also characterized by mutational analysis. Such interactions of these two GRPs may be important for function of either or both and therefore provide important information for further studies.

Exercise, heat shock proteins, and myocardial protection from I-R injury

Heat shock proteins (HSPs) play a critical role in maintaining cellular homeostasis and protecting cells during episodes of acute stress. Specifically, HSPs of the 70 kDa family (i.e., HSP72) are important in preventing ischemia-reperfusion induced apoptosis, necrosis, and oxidative injury in a variety of cell types including the cardiac myocyte. Evidence indicates that HSP72 may contribute to cellular protection against a variety of stresses by preventing protein aggregation, assisting in the refolding of damaged proteins, and chaperoning nascent polypeptides along ribosomes. Endurance exercise is a physiological stress that can be used to elevate myocardial levels of HSP72. It is now clear that endurance exercise training can elevate myocardial HSP72 by 400-500% in young adult animals. Importantly, an exercise-induced elevation in myocardial HSPs is associated with a reduction in ischemia-reperfusion (I-R) injury in the heart. Although it seems likely that exercise-induced elevations in myocardial levels of HSPs play an important role in this protection against an I-R insult, new evidence suggests that other factors may also be involved. This is an important area for future research.

The conserved carboxyl terminus and zinc finger-like domain of the co-chaperone Ydj1 assist Hsp70 in protein folding

Ydj1 is a member of the Hsp40 (DnaJ-related) chaperone family that facilitates cellular protein folding by regulating Hsp70 ATPase activity and binding unfolded polypeptides. Ydj1 contains four conserved subdomains that appear to represent functional units. To define the action of these regions, protease-resistant Ydj1 fragments and Ydj1 mutants were analyzed for activities exhibited by the unmodified protein. The Ydj1 mutant proteins analyzed were unable to support growth of yeast at elevated temperatures and were found to have alterations in the J-domain (Ydj1 H34Q), zinc finger-like region (Ydj1 C159T), and conserved carboxyl terminus (Ydj1 G315D). Fragment Ydj1 (1-90) contains the J-domain and a small portion of the G/F-rich region and could regulate Hsp70 ATPase activity but could not suppress the aggregation of the model protein rhodanese. Ydj1 H34Q could not regulate the ATPase activity of Hsp70 but could bind unfolded polypeptides. The J-domain functions independently and was sufficient to regulate Hsp70 ATPase activity. Fragment Ydj1 (179-384) could suppress rhodanese aggregation but was unable to regulate Hsp70. Ydj1 (179-384) contains the conserved carboxyl terminus of DnaJ but is missing the J-domain, G/F-rich region, and a major portion of the zinc finger-like region. Ydj1 G315D exhibited severe defects in its ability to suppress rhodanese aggregation and form complexes with unfolded luciferase. The conserved carboxyl terminus of Ydj1 appeared to participate in the binding of unfolded polypeptides. Ydj1 C159T could form stable complexes with unfolded proteins and suppress protein aggregation but was inefficient at refolding denatured luciferase. The zinc finger-like region of Ydj1 appeared to function in conjunction with the conserved carboxyl terminus to fold proteins. However, Ydj1 does not require an intact zinc finger-like region to bind unfolded polypeptides. These data suggest that the combined functions of the J-domain, zinc finger-like region, and the conserved carboxyl terminus are required for Ydj1 to cooperate with Hsp70 and facilitate protein folding in the cell.

Multiple ATP-dependent steps in RNA polymerase II promoter melting and initiation

Permanganate probing and abortive initiation assays were used to investigate the role of ATP in several successive stages of transcription initiation at the activated adeno E4 and mouse DHFR promoters. Removal of ATP at several points along the multi-step pathway blocked further progress towards its completion. Most strikingly, even if the DNA transcription start site is opened using ATP, the subsequent removal of ATP disallows formation of the first phosphodiester bond of the RNA. After ATP-dependent formation of a short RNA, a new transcription complex forms, which is more stable and has a longer open region. Both RNA and ATP appear to play roles in the formation of this complex. The need for ATP throughout this multi-step initiation pathway leads to new and unexpected possibilities for the use of energy and ATPases in transcription initiation.