title: Inhibition of protein synthesis by TOR inactivation revealed a conserved regulatory mechanism of the BiP chaperone in Chlamydomonas.
abstract: The target of rapamycin (TOR) kinase integrates nutritional and stress signals to coordinately control cell growth in all eukaryotes. TOR associates with highly conserved proteins to constitute two distinct signaling complexes termed TORC1 and TORC2. Inactivation of TORC1 by rapamycin negatively regulates protein synthesis in most eukaryotes. Here, we report that down-regulation of TOR signaling by rapamycin in the model green alga Chlamydomonas reinhardtii resulted in pronounced phosphorylation of the endoplasmic reticulum chaperone BiP. Our results indicated that Chlamydomonas TOR regulates BiP phosphorylation through the control of protein synthesis, since rapamycin and cycloheximide have similar effects on BiP modification and protein synthesis inhibition. Modification of BiP by phosphorylation was suppressed under conditions that require the chaperone activity of BiP, such as heat shock stress or tunicamycin treatment, which inhibits N-linked glycosylation of nascent proteins in the endoplasmic reticulum. A phosphopeptide localized in the substrate-binding domain of BiP was identified in Chlamydomonas cells treated with rapamycin. This peptide contains a highly conserved threonine residue that might regulate BiP function, as demonstrated by yeast functional assays. Thus, our study has revealed a regulatory mechanism of BiP in Chlamydomonas by phosphorylation/dephosphorylation events and assigns a role to the TOR pathway in the control of BiP modification.
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Científicas-Universidad de Sevilla, 41092 Seville, Spain.
authors.forename: Sandra
authors.initials: S
authors.lastname: Díaz-Troya
authors.forename: María Esther
authors.initials: ME
authors.lastname: Pérez-Pérez
authors.forename: Marta
authors.initials: M
authors.lastname: Pérez-Martín
authors.forename: Suzette
authors.initials: S
authors.lastname: Moes
authors.forename: Paul
authors.initials: P
authors.lastname: Jeno
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Florencio
authors.forename: José L
authors.initials: JL
authors.lastname: Crespo
mesh_terms: D001665:Binding Sites; D016825:Chlamydomonas reinhardtii / Q000187:drug effects / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D000091342:Endoplasmic Reticulum Chaperone BiP; D006031:Glycosylation / Q000187:drug effects; D006360:Heat-Shock Proteins / Q000378:metabolism*; D018869:Heat-Shock Response; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013912:Threonine; D014415:Tunicamycin / Q000494:pharmacology
chemical_list: D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D011500:Protein Synthesis Inhibitors; D014415:Tunicamycin; D013912:Threonine; D003513:Cycloheximide; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus