ER-associated degradation (ERAD) removes defective and mis-folded proteins from the eukaryotic secretory pathway, but mutations in the ER lumenal Hsp70, BiP/Kar2p, compromise ERAD efficiency in yeast. Because attenuation of ERAD activates the UPR, we screened for kar2 mutants in which the unfolded protein response (UPR) was induced in order to better define how BiP facilitates ERAD. Among the kar2 mutants isolated we identified the ERAD-specific kar2-1 allele (Brodsky et al. J. Biol. Chem. 274,(3453)(3454)(3455)(3456)(3457)(3458)(3459)(3460). The kar2-1 mutation resides in the peptide-binding domain of BiP and decreases BiP's affinity for a peptide substrate. Peptide-stimulated ATPase activity was also reduced, suggesting that the interdomain coupling in Kar2-1p is partially compromised. In contrast, Hsp40 cochaperone-activation of Kar2-1p's ATPase activity was unaffected. Consistent with UPR induction in kar2-1 yeast, an ERAD substrate aggregated in microsomes prepared from this strain but not from wild-type yeast. Overexpression of wild-type BiP increased substrate solubility in microsomes obtained from the mutant, but the ERAD defect was exacerbated, suggesting that simply retaining ERAD substrates in a soluble, retro-translocationcompetent conformation is insufficient to support polypeptide transit to the cytoplasm. INTRODUCTIONBefore being delivered to their ultimate locations, secreted proteins are monitored by a quality control "machine" associated with the endoplasmic reticulum (ER; reviewed by Ellgaard et al., 1999); aberrant polypeptides may be retrotranslocated from the ER to the cytoplasm and destroyed by the proteasome in a process termed ER-associated degradation (ERAD; McCracken and Brodsky, 1996). The importance of defining the molecular mechanism of ERAD is underscored by the fact that several human diseases arise from the accumulation or accelerated degradation of ERAD substrates and because some bacterial toxins and viruses coopt the ERAD pathway to exert their effects (reviewed in Thomas et al., 1995;Aridor and Hannan, 2000;Fewell et al., 2001).ERAD may result from inefficient protein folding, so it is not surprising that molecular chaperones are required for this process. Chaperones prevent the formation of off-pathway intermediates or directly catalyze folding and have been proposed to "judge" whether a nascent protein will ultimately fold or whether it should be targeted for degradation (reviewed by Hayes and Dice, 1996;Hartl, 1996;Horwich et al., 1999;Plemper and Wolf, 1999;Rö misch, 1999;Wickner et al., 1999;Fewell et al., 2001;Hö hfeld et al., 2001). Hsp70 (heat shock proteins with a molecular mass of ϳ70 kDa) molecular chaperones hydrolyze ATP concomitant with the binding of peptides with overall hydrophobic character (Flynn et al., 1991;Blond-Elguindi et al., 1993;Rü diger et al., 1997); therefore, Hsp70s might retain the solubility of unfolded, retro-translocating polypeptides during their voyage from the ER to the cytoplasm via the Sec61p translocation channel or might "gate" this c...

To determine whether the yeast Sec61p translocation pore is a high-affinity ribosome receptor in the endoplasmic reticulum, we isolated the Sec61p complex using an improved protocol in which contaminants found previously to be associated with the complex are absent. The purified complex, which contains Sec61p with an amino terminal hexahistidine tag, was active since it rescued a sec61-3 post-translational translocation defect in a reconstituted system. Co-reconstitution of the Sec61p and Sec63p complexes into liposomes failed to support post-translational translocation, suggesting that Sec62p is required for this process. By Scatchard analysis, the purified Sec61p complex bound to yeast ribosomes when reconstituted into liposomes with a K D of 5.6 nM, a value similar to the K D obtained when ribosome binding to total microsomal protein was measured (2.7 nM). In addition, a mammalian protein, p180, which has been proposed to be a ribosome receptor, was expressed in yeast, and endoplasmic reticulum-derived microsomes isolated from this strain exhibited ϳ2.3-fold greater binding to yeast ribosomes. Despite this increase in ribosome binding, neither co-nor post-translational translocation was compromised in vivo. In sum, our data suggest that the Sec61p complex is a ribosome receptor in the yeast endoplasmic reticulum membrane.

Since the secretory pathway is essential for Candida albicans to transition from a commensal organism to a pathogen, an understanding of how this pathway functions may be beneficial for identifying novel drug targets to prevent candidiasis. We have cloned the C. albicans KAR2 gene, which performs many roles during the translocation of proteins into the endoplasmic reticulum (ER) during the first committed step of the secretory pathway in many eukaryotes. Our results show that C. albicans KAR2 is essential, and that the encoded protein rescues a temperature-sensitive growth defect found in a Saccharomyces cerevisiae strain harboring a mutant form of the Kar2 protein. Additionally, S. cerevisiae containing CaKAR2 as the sole copy of this essential gene are viable, and ER microsomes prepared from this strain exhibit wild-type levels of post-translational translocation during in vitro translocation assays. Finally, ER microsomes isolated from a C. albicans strain expressing reduced amounts of KAR2 mRNA are defective for in vitro translocation of a secreted substrate protein, establishing a new method to study ER translocation in this organism. Together, these results suggest that C. albicans Kar2p functions during the translocation of proteins into the ER during the first step of the secretory pathway.

Nascent secretory proteins and integral membrane proteins must translocate into the endoplasmic reticulum (ER). The regulation of translocation remains ill‐defined. Post‐translational translocation into the yeast ER can be measured using ER‐derived microsomes or reconstituted proteoliposomes and as the substrate the mating pheromone precursor prepro‐alpha factor.

Since 2014, biology students at Fort Lewis College have studied the water quality of the Animas River in Durango, Colorado. Environmental microbiology and molecular biology techniques have been employed to study Escherichia coli isolates from the river and to define characteristics of the bacteria related to public health. E. coli was found in the river, as well as in culverts and tributary creeks that drain into the river within the Durango city limits. Concentrations of E. coli in the river occasionally exceeded the US EPA guideline of 126 CFU per 100 mL for recreational water use. Many of the E. coli isolates were able to be grown at 45 °C, an indication of mammalian origin. Unexpectedly, 8% of the isolates contained the intimin (eae) gene, a virulence gene characteristic of two pathotypes of E. coli, the enterohemorrhagic and enteropathogenic E. coli. Several isolates tested were resistant to multiple antibiotics commonly used in animal and human medicine. Further study is warranted to determine the source of these bacteria entering the Animas River, and to further characterize the possible disease potential of multi-antibiotic resistant and virulence gene-containing isolates found in a semi-rural/urban setting.