Environmental stresses inducing translation arrest are accompanied by the deposition of translational components into stress granules (SGs) serving as mRNA triage sites. It has recently been reported that, in Saccharomyces cerevisiae, formation of SGs occurs as a result of a prolonged glucose starvation. However, these SGs did not contain eIF3, one of hallmarks of mammalian SGs. We have analyzed the effect of robust heat shock on distribution of eIF3a/Tif32p/Rpg1p and showed that it results in the formation of eIF3a accumulations containing other eIF3 subunits, known yeast SG components and small but not large ribosomal subunits and eIF2α/Sui2p. Interestingly, under these conditions, Dcp2p and Dhh1p P-body markers also colocalized with eIF3a. Microscopic analyses of the edc3Δlsm4ΔC mutant demonstrated that different scaffolding proteins are required to induce SGs upon robust heat shock as opposed to glucose deprivation. Even though eIF2α became phosphorylated under these stress conditions, the decrease in polysomes and formation of SGs occurred independently of phosphorylation of eIF2α. We conclude that under specific stress conditions, such as robust heat shock, yeast SGs do contain eIF3 and 40S ribosomes and utilize alternative routes for their assembly.

Small heat shock proteins (sHsp) constitute an evolutionary conserved yet diverse family of chaperones acting as first line of defence against proteotoxic stress. sHsps coaggregate with misfolded proteins but the molecular basis and functional implications of these interactions, as well as potential sHsp specific differences, are poorly explored. In a comparative analysis of the two yeast sHsps, Hsp26 and Hsp42, we show in vitro that model substrates retain near-native state and are kept physically separated when complexed with either sHsp, while being completely unfolded when aggregated without sHsps. Hsp42 acts as aggregase to promote protein aggregation and specifically ensures cellular fitness during heat stress. Hsp26 in contrast lacks aggregase function but is superior in facilitating Hsp70/Hsp100-dependent post-stress refolding. Our findings indicate the sHsps of a cell functionally diversify in stress defence, but share the working principle to promote sequestration of misfolding proteins for storage in native-like conformation.

The large protein superfamily of NADPH oxidases (NOX enzymes) is found in members of all eukaryotic kingdoms: animals, plants, fungi, and protists. The physiological functions of these NOX enzymes range from defense to specialized oxidative biosynthesis and to signaling. In filamentous fungi, NOX enzymes are involved in signaling cell differentiation, in particular in the formation of fruiting bodies. On the basis of bioinformatics analysis, until now it was believed that the genomes of unicellular fungi like Saccharomyces cerevisiae and Schizosaccharomyces pombe do not harbor genes coding for NOX enzymes. Nevertheless, the genome of S. cerevisiae contains nine ORFs showing sequence similarity to the catalytic subunits of mammalian NOX enzymes, only some of which have been functionally assigned as ferric reductases involved in iron ion transport. Here we show that one of the nine ORFs (YGL160W, AIM14) encodes a genuine NADPH oxidase, which is located in the endoplasmic reticulum (ER) and produces superoxide in a NADPH-dependent fashion. We renamed this ORF YNO1 (yeast NADPH oxidase 1). Overexpression of YNO1 causes YCA1-dependent apoptosis, whereas deletion of the gene makes cells less sensitive to apoptotic stimuli. Several independent lines of evidence point to regulation of the actin cytoskeleton by reactive oxygen species (ROS) produced by Yno1p.cell cycle | integral membrane reductase | wiskostatin | latrunculin R eactive oxygen species (ROS) have multiple roles in physiology and pathophysiology, in particular during aging and induction of programmed cell death. This includes also nonmitochondrial sources, besides the long-studied mitochondrially generated ROS. These findings can be viewed as important additions to the classical "free radical theory of aging" (1) and theories developed thereafter (2, 3).In higher organisms, among others, at least two major sources of superoxide other than mitochondria are known. On the one hand, xanthine oxidase, an enzyme in the catabolism of purines, which catalyses the oxidation of hypoxanthine to xanthine and to uric acid, produces superoxide (4). On the other hand, NADPH oxidases (NOX) catalyze the production of superoxide from oxygen and NADPH (5).The NADPH oxidase superfamily of membrane-located enzymes of higher cells has been known for a decade (for review, ref. 5). Whereas the human NOX2 was discovered early on, other NOX (Nox1/3/4/5) as well as dual oxidase (DUOX) (Duox1/2) enzymes (displaying two domains: a NADPH oxidase domain and a peroxidase domain) have been found relatively recently in human cells. The human NOX2 was discovered as a defense enzyme of neutrophils and macrophages, which produce a burst of superoxide (O 2 · − ) as a first line of defense against invading microorganisms. Although X-ray or NMR structure determinations are not available, we know from indirect evidence and bioinformatics that the catalytic subunit of the macrophage enzyme contains six transmembrane helices, is located in the plasma membrane, and produces superoxide in a vectorial ...