No abstract

Until recently, the degradation of aberrant and unassembled proteins retained in the endoplasmic reticulum (ER) was thought to involve unidentified ER-localized proteases. We now show that the ER-associated degradation (ERAD) of two mutant proteins that accumulate in the ER lumen is inhibited in a proteasome-defective yeast strain and when cytosol from this mutant is used in an in vitro assay. In addition, ERAD is limited in vitro in the presence of the proteasome inhibitors, 3,4-dichloroisocoumarin and lactacystin. Furthermore, we find that an ERAD substrate is exported from ER-derived microsomes, and the accumulation of exported substrate is 2-fold greater when proteasome mutant cytosol is used in place of wild-type cytosol. We conclude that lumenal ERAD substrates are exported from the yeast ER to the cytoplasm for degradation by the proteasome complex.Quality control of newly synthesized proteins in the endoplasmic reticulum (ER) ensures that only correctly folded, processed, and completely assembled proteins exit this compartment for further transport through the secretory pathway. Most proteins that fail to reach this transport competent state are degraded (1-7). The ER-associated protein degradation pathway (ERAD) is highly selective for specific unassembled and͞or aberrant proteins, while the majority of ER resident and secreted proteins are quite stable. Evidence that the ER chaperone calnexin has a role in ERAD indicates that molecular chaperones might be required for this remarkable substrate selectivity (8).Although previous studies suggested that ERAD involves unidentified proteases localized in the ER, more recent evidence indicates that ERAD may require cytosolic proteases. Studies of ERAD in vitro suggest that degradation of aberrant ER-lumenal proteins occurs in the cytoplasm (8), and there is also evidence that the degradation of ER-retained forms of the cystic fibrosis transmembrane conductance regulator is ubiquitin-and proteasome-dependent (9, 10). In addition, cytomegalovirus-induced down-regulation of major histocompatibility complex class I molecules involves the rapid transport of the unassembled major histocompatibility complex I heavy chains from the ER to the cytoplasm for degradation by the proteasome complex (11). In this report we show that the cytosolic proteasome complex is the proteolytic component for the ER-associated degradation of two ER-lumenal proteins in yeast. Furthermore, we provide evidence that indicates ERAD substrates are exported to the cytosol for degradation by the proteasome. MATERIALS AND METHODSMaterials and Strains. Strains used were: RSY607 (Mat␣, pep4::URA3), provided by R. Schekman, University of California, Berkeley; AB122 (Mata, prc1-407, prb1-1122, pep4-3, leu2, ura3-52), provided by A. Brake, University of California, San Francisco; pre 1-1 pre 2-2 proteasome mutant and isogenic wild-type strain, provided by D. H. Wolf (12, 13); ⌬ubc6 ubiquitin-conjugating enzyme mutant and isogenic wild-type strains, provided by S. Jentsch (14); and ubc4-⌬1 u...

Endoglin (CD105), a transmembrane protein of the transforming growth factor ␤ superfamily, plays a crucial role in angiogenesis. Mutations in endoglin result in the vascular defect known as hereditary hemorrhagic telangiectasia (HHT1). The soluble form of endoglin was suggested to contribute to the pathogenesis of preeclampsia. To obtain further insight into its function, we cloned, expressed, purified, and characterized the extracellular domain (ECD) of mouse and human endoglin fused to an immunoglobulin Fc domain. We found that mouse and human endoglin ECD-Fc bound directly, specifically, and with high affinity to bone morphogenetic proteins 9 and 10 (BMP9 and BMP10) in surface plasmon resonance (Biacore) and cell-based assays. We performed a function mapping analysis of the different domains of endoglin by examining their contributions to the selectivity and biological activity of the protein. Endoglin (CD105) is a homodimeric glycosylated cell-surface protein of 180 kDa previously identified as a co-receptor belonging to the TGF␤ superfamily (1). Several lines of evidence support an important role of endoglin in cardiovascular development and vascular remodeling (2). Loss-of-function mutations in endoglin are implicated in the vascular disorder hereditary hemorrhagic telangiectasia type 1 (HHT1), 3 which is a bleeding disorder characterized by arteriovenous malformations in the brain, lungs, and liver and is attributed to haploinsufficiency (3-5). Homozygous endoglin knock-out mice die during early gestation due to the lack of development of normal mature blood vessels (6). Adult endoglin heterozygous mice (7-8) or mice with a conditional mutation in the endoglin gene (9) exhibited similar angiogenic abnormalities and were used as animal models for HHT1. Mutations in the endoglin gene found in numerous HHT1 patients are localized most exclusively in the extracellular domain (4). The specific role of endoglin in the vascular dysplasia observed in HHT patients is not known, but it is likely to be related to the role of TGF␤ family signaling in angiogenesis (2, 10). Interestingly, another form of HHT, known as HHT2, which is also characterized by the presence of telangiectases as well as arteriovenous malformations in brain, lungs and liver, results from the loss of TGF␤ type I receptor ALK1 (11), which suggests an interrelatedness between endoglin and ALK1 and possibly involvement of the same ligand(s) in the mechanism of action of both molecules.A soluble form of endoglin has been observed in the serum of patients with different types of solid malignancies (12) and of pregnant women suffering from preeclampsia, a disease leading to vascular permeability (13), hypertension, and placental abruption (14). This soluble form, which reportedly results from partial shedding of the membrane-bound form of endoglin by the matrix metalloproteinase 14 (MT1-MMP) (15), a phenomenon also observed for the other type III receptor betaglycan (16), has been proposed to act as a scavenger or trap for circulating TGF␤ family liga...

The nuclear lamins form a two-dimensional matrix that provides integrity to the cell nucleus and participates in nuclear activities. Mutations in the region of human LMNA encoding the carboxyl-terminal tail Lamin A/C are associated with forms of muscular dystrophy and familial partial lipodystrophy (FPLD). To help discriminate tissue-specific phenotypes, we have solved at 1.4-Å resolution the three-dimensional crystal structure of the lamin A/C globular tail. The domain adopts a novel, all ␤ immunoglobulin-like fold. FPLD-associated mutations cluster within a small surface, whereas muscular dystrophyassociated mutations are distributed throughout the protein core and on its surface. These findings distinguish myopathy-and lipodystrophy-associated mutations and provide a structural framework for further testing hypotheses concerning lamin function.

Inhibitory serine phosphorylation is a potential molecular mechanism for insulin resistance. We have developed a new variant of the yeast two-hybrid method, referred to as disruptive yeast tri-hybrid (Y3H), to identify inhibitory kinases and sites of phosphorylation in insulin receptors (IR) and IR substrates, IRS-1. Using IR and IRS-1 as bait and prey, respectively, and c-Jun NH 2 -terminal kinase (JNK1) as the disruptor, we now show that phosphorylation of IRS-1 Ser-307, a previously identified site, is necessary but not sufficient for JNK1-mediated disruption of IR/IRS-1 binding. We further identify a new phosphorylation site, Ser-302, and show that this too is necessary for JNK1-mediated disruption. Seven additional kinases potentially linked to insulin resistance similarly block IR/IRS-1 binding in the disruptive Y3H, but through distinct Ser-302-and Ser-307-independent mechanisms. Phosphospecific antibodies that recognize sequences surrounding Ser(P)-302 or Ser(P)-307 were used to determine whether the sites were phosphorylated under relevant conditions. Phosphorylation was promoted at both sites in Fao hepatoma cells by reagents known to promote Ser/Thr phosphorylation, including the phorbol ester phorbol 12-myristate 13-acetate, anisomycin, calyculin A, and insulin. The antibodies further showed that Ser(P)-302 and Ser(P)-307 are increased in animal models of obesity and insulin resistance, including genetically obese ob/ob mice, dietinduced obesity, and upon induction of hyperinsulinemia. These findings demonstrate that phosphorylation at both Ser-302 and Ser-307 is necessary for JNK1-mediated inhibition of the IR/IRS-1 interaction and that Ser-302 and Ser-307 are phosphorylated in parallel in cultured cells and in vivo under conditions that lead to insulin resistance.

Polypeptide import into the yeast endoplasmic reticulum (ER) requires two hsp70s, Ssa1p in the cytosol and BiP (Kar2p) in the ER lumen. After import, aberrant polypeptides may be exported to the cytoplasm for degradation by the proteasome, and defects in the ER chaperone calnexin (Cne1p) compromise their degradation. Both import and export require BiP and the Sec61p translocation complex, suggesting that import and export may be mechanistically related. We now show that the cne1⌬ and two kar2 mutant alleles exhibit a synthetic interaction and that the export and degradation of pro-␣ factor is defective in kar2 mutant microsomes. Pulse-chase analysis indicates that A1PiZ, another substrate for degradation, is stabilized in the kar2 strains at the restrictive temperature. Because two of the kar2 mutants examined are proficient for polypeptide import, the roles of BiP during ER protein export and import differ, indicating that these processes must be mechanistically distinct. To examine whether Ssa1p drives polypeptides from the ER and is also required for degradation, we assembled reactions using strains either containing a mutation in SSA1 or in which the level of Ssa1p could be regulated. We found that pro-␣ factor and A1PiZ were degraded normally, indicating further that import and export are distinct and that other cytosolic factors may pull polypeptides from the ER.

Diseases that affect the regulation of bone turnover can lead to skeletal fragility and increased fracture risk. Members of the TGF-␤ superfamily have been shown to be involved in the regulation of bone mass. Activin A, a TGF-␤ signaling ligand, is present at high levels in bone and may play a role in the regulation of bone metabolism. Here we demonstrate that pharmacological blockade of ligand signaling through the high affinity receptor for activin, type II activin receptor (ActRIIA), by administration of the soluble extracellular domain of ActRIIA fused to a murine IgG2a-Fc, increases bone formation, bone mass, and bone strength in normal mice and in ovariectomized mice with established bone loss. These observations support the development of this pharmacological strategy for the treatment of diseases with skeletal fragility.anabolic ͉ osteoporosis ͉ TGF-␤ ͉ therapeutic

Grb10 has been proposed to inhibit or activate insulin signaling, depending on cellular context. We have investigated the mechanism by which full-length hGrb10␥ inhibits signaling through the insulin receptor substrate (