SUMO conjugation to proteins is involved in the regulation of diverse cellular functions. We have identified a protein, RWD-containing sumoylation enhancer (RSUME), that enhances overall SUMO-1, -2, and -3 conjugation by interacting with the SUMO conjugase Ubc9. RSUME increases noncovalent binding of SUMO-1 to Ubc9 and enhances Ubc9 thioester formation and SUMO polymerization. RSUME enhances the sumoylation of IkB in vitro and in cultured cells, leading to an inhibition of NF-kB transcriptional activity. RSUME is induced by hypoxia and enhances the sumoylation of HIF-1alpha, promoting its stabilization and transcriptional activity during hypoxia. Disruption of the RWD domain structure of RSUME demonstrates that this domain is critical for RSUME action. Together, these findings point to a central role of RSUME in the regulation of sumoylation and, hence, several critical regulatory pathways in mammalian cells.

Asparagine-linked glycosylation is a highly conserved protein modification reaction that occurs in all eukaryotes. The initial stage in the biosynthesis of N-linked glycoproteins, catalyzed by the enzyme oligosaccharyltransferase (OST), involves the transfer of a preassembled high-mannose oligosaccharide from a dolichol-linked oligosaccharide donor onto asparagine acceptor sites in nascent proteins in the lumen of the rough endoplasmic reticulum. Biochemical, molecular biological, and genetic studies conducted during the past 5 years have resulted in an explosive growth in our knowledge concerning the OST. Although the basic biochemical properties of the enzyme were determined more than a decade ago using intact microsomal membranes, recent studies provide novel insight into the catalytic mechanism of the enzyme. The OST was recently purified as a large heteroligomeric membrane protein complex; the sequences of many of the subunits have been determined from both fungal and vertebrate sources. Consistent with the evolutionary conservation of N-linked glycosylation, protein sequence comparisons reveal significant homologies between vertebrate, invertebrate, plant, and fungal OST subunits. Yeast molecular genetic methods have been instrumental in the functional characterization of the OST subunits, and have proven to be powerful tools for the identification of novel gene products that influence oligosaccharide transfer in vivo.

G protein–coupled receptor CRHR1 activates both soluble adenylyl cyclase (sAC) and transmembrane adenylyl cyclases. Here, Inda et al. show that only sAC activity is essential for internalization-dependent cAMP and sustained ERK1/2 activation responses, revealing a functional association between sAC-generated cAMP and endosome-based G protein–coupled receptor signaling.

Abstract. Oligosaccharyltransferase catalyzes the transfer of a preassembled high mannose oligosaccharide from a dolichol-oligosaccharide donor to consensus glycosylation acceptor sites in newly synthesized proteins in the lumen of the rough endoplasmic reticulum. The Saccharomyces cerevisiae oligosaccharyltransferase is an oligomeric complex composed of six nonidentical subunits (c~-~). The a, [3, y, and 8 PARAGINE-linked glycosylation of proteins is a highly conserved protein modification reaction that occurs in the lumen of the rough endoplasmic reticulum in all eukaryotic organisms (Kornfeld and Kornfeld, 1985;Herscovics and Orlean, 1993). The initial stage in the biosynthesis of N-glycosylated proteins, catalyzed by the lumenally oriented enzyme oligosaccharyltransferase, involves the transfer of a preassembled high-mannose oligosaccharide (Glc3Man9GlcNAc2) from a dolichol-pyrophosphate donor onto asparagine acceptor sites within the consensus sequon Asn-X-Ser/Thr, where X can be any amino acid except proline (Gavel and Von Heijne, 1990). N-linked glycosylation is an obligatory event for the efficient folding and oligomeric assembly of many nascent

Members of the eukaryotic heat shock protein 70 family (Hsp70s) are regulated by protein cofactors that contain domains homologous to bacterial DnaJ. Of the three DnaJ homologues in the yeast rough endoplasmic reticulum (RER; Scj1p, Sec63p, and Jem1p), Scj1p is most closely related to DnaJ, hence it is a probable cofactor for Kar2p, the major Hsp70 in the yeast RER. However, the physiological role of Scj1p has remained obscure due to the lack of an obvious defect in Kar2p-mediated pathways in scj1 null mutants. Here, we show that the Δscj1 mutant is hypersensitive to tunicamycin or mutations that reduce N-linked glycosylation of proteins. Although maturation of glycosylated carboxypeptidase Y occurs with wild-type kinetics in Δscj1 cells, the transport rate for an unglycosylated mutant carboxypeptidase Y (CPY) is markedly reduced. Loss of Scj1p induces the unfolded protein response pathway, and results in a cell wall defect when combined with an oligosaccharyltransferase mutation. The combined loss of both Scj1p and Jem1p exaggerates the sensitivity to hypoglycosylation stress, leads to further induction of the unfolded protein response pathway, and drastically delays maturation of an unglycosylated reporter protein in the RER. We propose that the major role for Scj1p is to cooperate with Kar2p to mediate maturation of proteins in the RER lumen.

Abstract. Oligosaccharyltransferase mediates the transfer of a preassembled high mannose oligosaccharide from a lipid-linked oligosaccharide donor to consensus glycosylation acceptor sites in newly synthesized proteins in the lumen of the rough endoplasmic reticulum. The Saccharomyces cerevisiae oligosaccharyltransferase is an oligomeric complex composed of six nonidentical subunits (a-D, two of which are glycoproteins (a and B)-The B and 8 subunits of the oligosaccharyltransferase are encoded by the WBP1 and SWP1 genes. Here we describe the functional characterization of the OS/7 gene that encodes the ot subunit of the oligosaccharyltransferase. Protein sequence analysis revealed a significant sequence identity between the Saccharomyces cerevisiae Ostl protein and ribophorin I, a previ.ously identified subunit of the mammalian oligosaccharyltransferase. A disruption of the OS'H locus was not tolerated in haploid yeast showing that expression of the Ostl protein is essential for vegetative growth of yeast. An analysis of a series of conditional ostl mutants demonstrated that defects in the Ostl protein cause pleiotropic underglycosylation of soluble and membrane-bound glycoproteins at both the permissive and restrictive growth temperatures. Microsomal membranes isolated from ostl mutant yeast show marked reductions in the in vitro transfer of high mannose oligosaccharide from exogenous lipid-linked oligosaccharide to a glycosylation site acceptor tripeptide. Microsomal membranes isolated from the ostl mutants contained elevated amounts of the Kar2 stress-response protein.PARAGINE-Iinked glycosylation of proteins is a ubiquitous protein modification reaction in eukaryotic organisms that occurs in the lumen of the rough endoplasmic reticulum (Herscovics and Orlean, 1993;Kornfeld and Kornfeld, 1985). Addition of asparagine-linked carbohydrates to many glycoproteins is an obligatory event for folding and assembly of newly synthesized polypeptides (Helenius, 1994). The presence of oligosaccharides is often required for the efficient transport of individual glycoproteins through the secretory pathway (Guan et al., 1985;Riederer and Hinnen, 1991;Winther et al., 1991). Glycan groups contribute to the overall dynamic stability of proteins, in some cases rendering them more resistant to proteolysis in vivo (Barriocanal et al., 1986). Diverse biological roles for asparagine-linked oligosaccharides have been identified including serving as receptors for extracellular

The fungal pathogen Candida albicans switches from a yeast-like to a filamentous mode of growth in response to a variety of environmental conditions. We examined the morphogenetic behavior of C. albicans yeast cells lacking the BCY1 gene, which encodes the regulatory subunit of protein kinase A. We cloned the BCY1 gene and generated a bcy1 tpk2 double mutant strain because a homozygous bcy1 mutant in a wild-type genetic background could not be obtained. In the bcy1 tpk2 mutant, protein kinase A activity (due to the presence of the TPK1 gene) was cyclic AMP independent, indicating that the cells harbored an unregulated phosphotransferase activity. This mutant has constitutive protein kinase A activity and displayed a defective germinative phenotype in N-acetylglucosamine and in serum-containing medium. The subcellular localization of a Tpk1-green fluorescent protein (GFP) fusion protein was examined in wild-type, tpk2 null, and bcy1 tpk2 double mutant strains. The fusion protein was observed to be predominantly nuclear in wild-type and tpk2 strains. This was not the case in the bcy1 tpk2 double mutant, where it appeared dispersed throughout the cell. Coimmunoprecipitation of Bcy1p with the Tpk1-GFP fusion protein demonstrated the interaction of these proteins inside the cell. These results suggest that one of the roles of Bcy1p is to tether the protein kinase A catalytic subunit to the nucleus.Candida albicans is an opportunistic human fungal pathogen of great medical significance in immunocompromised patients (25). This fungus has the capability of switching its mode of growth between budding yeast and hypha or pseudohypha in response to environmental signals. Genetic evidence indicates that the morphogenetic switch to the hyphal mode of growth, though associated with pathogenicity and virulence (20), is necessary but not sufficient to trigger disease (5). The relationship between morphology and pathogenicity has been the focus of intensive research devoted to the study of the developmental programs involved in the dimorphic transition.The remarkable conservation of signal transduction pathways in fungi allowed the identification of components of these pathways in several fungal species based on the insight gained from studying pseudohyphal differentiation in Saccharomyces cerevisiae. In C. albicans, two major pathways implicated in dimorphism could be established: the mitogen-activated protein kinase and the cyclic AMP (cAMP)/protein kinase A transduction pathways (for a review, see reference 19).Initial biochemical studies indicate that high cAMP levels promote the yeast-to-hypha transition in C. albicans (23,31). In addition, we have shown that in vivo inhibition of protein kinase A blocks hyphal growth induced by N-acetylglucosamine (GlcNAc) (6). Recent genetic studies allowed the identification of the genes involved in the cAMP/protein kinase A pathway. A transduction cascade similar to that of S. cerevisiae, with regard to location and function of the homologous components, has been established. Thus, CaRa...

CRH is a key regulator of neuroendocrine, autonomic, and behavioral response to stress. CRH-stimulated CRH receptor 1 (CRHR1) activates ERK1/2 depending on intracellular context. In a previous work, we demonstrated that CRH activates ERK1/2 in limbic areas of the mouse brain (hippocampus and basolateral amygdala). ERK1/2 is an essential mediator of hippocampal physiological processes including emotional behavior, synaptic plasticity, learning, and memory. To elucidate the molecular mechanisms by which CRH activates ERK1/2 in hippocampal neurons, we used the mouse hippocampal cell line HT22. We document for the first time that ERK1/2 activation in response to CRH is biphasic, involving a first cAMP- and B-Raf-dependent early phase and a second phase that critically depends on CRHR1 internalization and β-arrestin2. By means of mass-spectrometry-based screening, we identified B-Raf-associated proteins that coimmunoprecipitate with endogenous B-Raf after CRHR1 activation. Using molecular and pharmacological tools, the functional impact of selected B-Raf partners in CRH-dependent ERK1/2 activation was dissected. These results indicate that 14-3-3 proteins, protein kinase A, and Rap1, are essential for early CRH-induced ERK1/2 activation, whereas dynamin and vimentin are required for the CRHR1 internalization-dependent phase. Both phases of ERK1/2 activation depend on calcium influx and are affected by calcium/calmodulin-dependent protein kinase II inactivation. Thus, this report describes the dynamics and biphasic nature of ERK1/2 activation downstream neuronal CRHR1 and identifies several new critical components of the CRHR1 signaling machinery that selectively controls the early and late phases of ERK1/2 activation, thus providing new potential therapeutic targets for stress-related disorders.