Biomedical subjects

Tomoki Chiba

Publications and source records attributed to Tomoki Chiba.

39 records · Page 3Linked to original sources

PubMed2002

E3 ubiquitin ligase that recognizes sugar chains.

N-glycosylation of proteins in the endoplasmic reticulum (ER) has a central role in protein quality control. Here we report that N-glycan serves as a signal for degradation by the Skp1-Cullin1-Fbx2-Roc1 (SCF(Fbx2)) ubiquitin ligase complex. The F-box protein Fbx2 (ref. 4) binds specifically to proteins attached to N-linked high-mannose oligosaccharides and subsequently contributes to ubiquitination of N-glycosylated proteins. Pre-integrin beta 1 is a target of Fbx2; these two proteins interact in the cytosol after inhibition of the proteasome. In addition, expression of the mutant Fbx2 Delta F, which lacks the F-box domain that is essential for forming the SCF complex, appreciably blocks degradation of typical substrates of the ER-associated degradation pathway. Our results indicate that SCF(Fbx2) ubiquitinates N-glycosylated proteins that are translocated from the ER to the cytosol by the quality control mechanism.

Animals↗

PubMed2002

Two distinct pathways mediated by PA28 and hsp90 in major histocompatibility complex class I antigen processing.

Major histocompatibility complex (MHC) class I ligands are mainly produced by the proteasome. Herein, we show that the processing of antigens is regulated by two distinct pathways, one requiring PA28 and the other hsp90. Both hsp90 and PA28 enhanced the antigen processing of ovalbumin (OVA). Geldanamycin, an inhibitor of hsp90, almost completely suppressed OVA antigen presentation in PA28alpha(-/-)/beta(-/-) lipopolysaccharide blasts, but not in wild-type cells, indicating that hsp90 compensates for the loss of PA28 and is essential in the PA28-independent pathway. In contrast, treatment of cells with interferon (IFN)-gamma, which induces PA28 expression, abrogated the requirement of hsp90, suggesting that IFN-gamma enhances the PA28-dependent pathway, whereas it diminishes hsp90-dependent pathway. Importantly, IFN-gamma did not induce MHC class I expressions in PA28-deficient cells, indicating a prominent role for PA28 in IFN-gamma-stimulated peptide supply. Thus, these two pathways operate either redundantly or specifically, depending on antigen species and cell type.

Amino Acid Sequence↗

PubMed2002

Antagonistic regulation of myogenesis by two deubiquitinating enzymes, UBP45 and UBP69.

Protein modification by ubiquitin is a dynamic and reversible process that is involved in the regulation of a variety of cellular processes. Here, we show that myogenic differentiation of embryonic muscle cells is antagonistically regulated by two deubiquitinating enzymes, UBP45 and UBP69, that are generated by alternative splicing. Both enzymes cleaved off ubiquitin from polyubiquitinated protein conjugates in vivo as well as from linear ubiquitin-protein fusions in vitro. In cultured myoblasts, the level of UBP69 mRNA markedly but transiently increased before membrane fusion, whereas that of UBP45 mRNA increased as the cells fused to form myotubes. Both myoblast fusion and accumulation of myosin heavy chain were dramatically stimulated by the stable expression of UBP69 but strongly attenuated by that of the catalytically inactive form of the protease, suggesting that the mutant enzyme acts dominant negatively on the function of the wild-type protease. In contrast, stable expression of UBP45 completely blocked both of the myogenic processes but that of inactive enzyme did not, indicating that the catalytic activity of the enzyme is essential for its inhibitory effects. These results indicate that differential expression of UBP45 and UBP69 is involved in the regulation of muscle cell differentiation.

Animals↗