Secretory and transmembrane proteins enter the endoplasmic reticulum (ER) as unfolded proteins and exit as either folded proteins in transit to their target organelles or as misfolded proteins targeted for degradation. The unfolded protein response (UPR) maintains the protein-folding homeostasis within the ER, ensuring that the protein-folding capacity of the ER meets the load of client proteins. Activation of the UPR depends on three ER stress sensor proteins, Ire1, PERK, and ATF6. Although the consequences of activation are well understood, how these sensors detect ER stress remains unclear. Recent evidence suggests that yeast Ire1 directly binds to unfolded proteins, which induces its oligomerization and activation. BiP dissociation from Ire1 regulates this oligomeric equilibrium, ultimately modulating Ire1's sensitivity and duration of activation. The mechanistic principles of ER stress sensing are the focus of this review.

The protein ␣-synuclein has a central role in Parkinson disease, but the mechanism by which it contributes to neural degeneration remains unknown. We now show that the expression of ␣-synuclein in mammalian cells, including neurons in vitro and in vivo, causes the fragmentation of mitochondria. The effect is specific for synuclein, with more fragmentation by ␣-than ␤-or ␥-isoforms, and it is not accompanied by changes in the morphology of other organelles or in mitochondrial membrane potential. However, mitochondrial fragmentation is eventually followed by a decline in respiration and neuronal death. The fragmentation does not require the mitochondrial fission protein Drp1 and involves a direct interaction of synuclein with mitochondrial membranes. In vitro, synuclein fragments artificial membranes containing the mitochondrial lipid cardiolipin, and this effect is specific for the small oligomeric forms of synuclein. ␣-Synuclein thus exerts a primary and direct effect on the morphology of an organelle long implicated in the pathogenesis of Parkinson disease.Many observations have implicated mitochondria in the pathogenesis of PD.2 Mitochondria from the substantia nigra of affected patients show a selective reduction in the activity of respiratory chain complex I (1). Somatic mutations also accumulate with age and PD in the mitochondrial DNA of substantia nigra neurons (2). In addition, the neurotoxins MPTP and rotenone, which produce models of PD, both act by disrupting mitochondrial function. Genetic evidence further supports a primary role for mitochondria in the pathogenesis of PD. Mutations in parkin and the mitochondrial kinase PINK1 both cause autosomal recessive PD (3), and these genes appear required for the normal clearance of defective mitochondria by autophagy (4). However, the molecular mechanisms responsible for mitochondrial dysfunction in the much more common sporadic forms of PD have remained unclear. Several observations suggest a central role for the protein ␣-synuclein in the pathogenesis of sporadic PD. Point mutations in synuclein produce a rare autosomal dominant form of PD (5-7), indicating a causative role for the protein. ␣-Synuclein also accumulates in the Lewy bodies and dystrophic neurites of essentially all patients with idiopathic PD (8), implicating the protein in sporadic as well as familial forms of the disease. Furthermore, duplication and particularly triplication of the SNCA (␣-synuclein) gene cause a severe, highly penetrant form of PD (9, 10), indicating a dose-dependent pathogenic role for the wild type protein when overexpressed and suggesting that the accumulation of synuclein in sporadic PD is the primary pathogenic event. However, the mechanism by which ␣-synuclein causes PD remains poorly understood. Expressed in yeast and Drosophila, human ␣-synuclein produces severe toxicity (11-14), but these model organisms lack endogenous synuclein, and the overexpression of wild type synuclein in mammalian systems causes remarkably little if any consistent toxicity (15-18).Althou...

The unfolded protein response (UPR) detects the accumulation of unfolded proteins in the endoplasmic reticulum (ER) and adjusts the protein folding capacity to the needs of the cell. Under conditions of ER stress, the transmembrane protein Ire1 oligomerizes to activate its cytoplasmic kinase and RNase domains. It is unclear what feature of ER stress Ire1 detects.

Pex1 and Pex6 are Type-2 AAA+ ATPases required for the de-novo biogenesis of peroxisomes. Mutations in Pex1 and Pex6 account for the majority of the most severe forms of peroxisome biogenesis disorders in humans. Here we show that the ATP-dependent complex of Pex1 and Pex6 from S. cerevisiae is a heterohexamer with alternating subunits. Within the Pex1/Pex6 complex, only the D2 ATPase ring hydrolyzes ATP, while nucleotide binding in the D1 ring promotes complex assembly. ATP hydrolysis by Pex1 is highly coordinated with that of Pex6. Furthermore, Pex15, the membrane anchor required for Pex1/Pex6 recruitment to peroxisomes inhibits the ATP-hydrolysis activity of Pex1/Pex6.