The chaperone protein network controls both initial protein folding and subsequent maintenance of proteins in the cell. Although the native structure of a protein is principally encoded in its amino-acid sequence, the process of folding in vivo very often requires the assistance of molecular chaperones. Chaperones also play a role in a post-translational quality control system and thus are required to maintain the proper conformation of proteins under changing environmental conditions. Many factors leading to unfolding and misfolding of proteins eventually result in protein aggregation. Stress imposed by high temperature was one of the first aggregation-inducing factors studied and remains one of the main models in this field. With massive protein aggregation occurring in response to heat exposure, the cell needs chaperones to control and counteract the aggregation process. Elimination of aggregates can be achieved by solubilization of aggregates and either refolding of the liberated polypeptides or their proteolysis. Here, we focus on the molecular mechanisms by which heat-shock protein 70 (Hsp70), Hsp100 and small Hsp chaperones liberate and refold polypeptides trapped in protein aggregates.

Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease. Motor neuron diseases (MNDs), including ALS and spastic paraplegia (SP), are characterized by the progressive, lengthdependent degeneration of motor neurons, leading to muscle atrophy, paralysis, and, in some cases, premature death. There are both inherited and sporadic forms of MNDs, which can affect upper motor neurons, lower motor neurons, or both. Although the molecular and cellular causes of most MNDs are unknown, many are associated with defects in axonal transport of cellular components required for neuron function and maintenance (1-6).A subset of MNDs is associated with impaired mitochondrial respiration and mitochondrial distribution. This observation has led to the hypothesis that neurodegeneration results from defects in mitochondrial motility and distribution, which, in turn, cause subcellular ATP depletion and interfere with mitochondrial calcium ([Ca 2+ ] m ) buffering at sites of high synaptic activity (reviewed in ref. 7). It is not known, however, whether mitochondrial motility defects are a primary cause or a secondary consequence of MND progression. In addition, it has been difficult to isolate the primary effect of mitochondrial motility defects in MNDs because most mutations that impair mitochondrial motility in neurons also affect transport of other organelles and vesicles (1,(8)(9)(10)(11).In mammals, the movement of neuronal mitochondria between the cell body and the synapse is controlled by adaptors called trafficking kinesin proteins (Trak1 and Trak2) and molecular motors (kinesin heavy chain and dynein), which transport the organelle in the anterograde or retrograde direction along axonal microtubule tracks (7,(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). Mitochondrial Rho (Miro) GTPase proteins are critical for transport because they are the only known surface receptors that attach mitochondria to...

The ratio of ATP:ADP is highest at perinuclear sites, where mitochondria are dense, and dissipates toward the periphery. Miro1 positions mitochondria toward the cortical cytoskeleton. Deletion of Miro1 results in perinuclear clustering of mitochondria, altering intracellular ATP:ADP gradients, and impairs energy-expensive cell migratory processes.

In yeast, a protein complex termed the ER-Mitochondria Encounter Structure (ERMES) tethers mitochondria to the endoplasmic reticulum. ERMES proteins are implicated in a variety of cellular functions including phospholipid synthesis, mitochondrial protein import, mitochondrial attachment to actin, polarized mitochondrial movement into daughter cells during division, and maintenance of mitochondrial DNA (mtDNA). The mitochondrial-anchored Gem1 GTPase has been proposed to regulate ERMES functions. Here, we show that ERMES and Gem1 have no direct role in the transport of phosphatidylserine (PS) from the ER to mitochondria during the synthesis of phosphatidylethanolamine (PE), as PS to PE conversion is not affected in ERMES or gem1 mutants. In addition, we report that mitochondrial inheritance defects in ERMES mutants are a secondary consequence of mitochondrial morphology defects, arguing against a primary role for ERMES in mitochondrial association with actin and mitochondrial movement. Finally, we show that ERMES complexes are long-lived, and do not depend on the presence of Gem1. Our findings suggest that the ERMES complex may have primarily a structural role in maintaining mitochondrial morphology.

Formation of β-hairpins is considered the initial step of folding of many proteins and, consequently, peptides constituting the β-hairpin sequence of proteins (the β-hairpin-forming peptides) are considered as models of early stages of protein folding. In this article, we discuss the results of experimental studies (circular-dichroism, infrared and nuclear magnetic resonance spectroscopy, and differential scanning calorimetry) of the structure of β-hairpin-forming peptides excised from the B1 domain of protein G, which are known to fold on their own. We demonstrate that local interactions at the turn sequence and hydrophobic interactions between nonpolar residues are the dominant structure-determining factors, while there is no convincing evidence that stable backbone hydrogen bonds are formed in these peptides in aqueous solution. Consequently, the most plausible mechanism for folding of the β-hairpin sequence appears to be the broken-zipper mechanism consisting of the following three steps: (i) bending the chain at the turn sequence owing to favorable local interactions, (ii) formation of loose hydrophobic contacts between nonpolar residues, which occur close to the contacts in the native structure of the protein but not exactly in the same position and, finally, (iii) formation of backbone hydrogen bonds and locking the hydrophobic contacts in the native positions as a hydrophobic core develops, sufficient to dehydrate the backbone peptide groups. This mechanism provides sufficient uniqueness (contacts form between residues that become close together because the chain is bent at the turn position) and robustness (contacts need not occur at once in the native positions) for folding a β-hairpin sequence.