Quality control in the endoplasmic reticulum: Folding and misfolding of vesicular stomatitis virus G protein in cells and in vitro

Parallel experiments in living cells and in vitro were undertaken to characterize the mechanism by which misfolded and unassembled glycoproteins are retained in the ER. A thermoreversible folding mutant of vesicular stomatitis virus (VSV) G protein called ts045 was analyzed. At 39°C, newly synthesized G failed to fold correctly according to several criteria: intrachain disulfide bonds were incomplete; the B2 epitope was absent; and the protein was associated with immunoglobulin heavy chain binding protein (BiP), a heat shock-related, ER protein. When the temperature was lowered to 32°C, these properties were reversed, and the protein was transported to the cell surface. Upon the shift up from 32°C back to 39°C, G protein in the ER returned to the misfolded form and was retained, while the protein that had reached a pre-Golgi compartment or beyond was thermostable and remained transport competent. The misfolding reaction could be reconstituted in a cell free system using ts045 virus particles and protein extracts from microsomes. Taken together, the results showed that ER is unique among the organelles of the secretory pathway in containing specific factors capable of misfolding G protein at the nonpermissive temperature and thus participating in its retention

The hyperthermophilic anaerobe Thermotoga Maritima is able to cope with limited amount of oxygen : insights into its defence strategies

Thermotoga maritima, an anaerobic hyperthermophilic bacterium, was found able to grow in the presence of low concentrations of oxygen of up to 0.5% (v/v). Differential proteomics and transcripts analysis by qRT-PCR were used to identify the defence strategies used by T. maritima to protect itself against oxygen. A flavoprotein, homologous to rubredoxin oxygen oxidoreductase was found to be overproduced when cells were cultured in oxidative conditions. The recombinant protein, produced in Escherichia coli, exhibited an oxygen reductase activity, which could account for the observed decrease in oxygen concentration during growth. The gene encoding this oxygen reductase belongs to a multicistronic unit that includes genes encoding proteins involved in exopolysaccharide biosynthesis, which may be related to a biofilm formation induced by the presence of oxygen. Enzymes involved in reactive oxygen species detoxification, iron-sulfur centre synthesis/repair and the cysteine biosynthesis pathway were also overproduced. All these enzymatic systems together contribute to the defence strategy of T. maritima against oxygen. Because of the position of T. maritima in deep branches of the phylogenetic tree, we suggest that these strategies can be considered as ancestral mechanisms first developed by anaerobic microorganisms on the early Earth to protect themselves against primary abiotic or biotic oxygen production