Inhibition of Protein Synthesis by TOR Inactivation Revealed a Conserved Regulatory Mechanism of the BiP Chaperone in Chlamydomonas

• Published in: Plant physiology (Bethesda) 2011-10, Vol.157 (2), p.730-741
• Main Authors: Díaz-Troya, Sandra, Pérez-Pérez, María Esther, Pérez-Martín, Marta, Moes, Suzette, Jeno, Paul, Florencio, Francisco J., Crespo, José L.
• Format: Article
• Language: English
• Subjects: Antibodies; Binding Sites; Biological and medical sciences; Cell growth; Chlamydomonas; Chlamydomonas reinhardtii; Chlamydomonas reinhardtii - drug effects; Chlamydomonas reinhardtii - metabolism; Cycloheximide - pharmacology; ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS; Eukaryotic cells; Fundamental and applied biological sciences. Psychology; Gels; Gene expression regulation; Glycosylation - drug effects; Heat-Shock Proteins - metabolism; Heat-Shock Response; Phosphorylation; Plant cells; Plant physiology and development; Plants; Protein Biosynthesis - drug effects; Protein synthesis; Protein Synthesis Inhibitors - pharmacology; Sirolimus - pharmacology; Threonine; TOR Serine-Threonine Kinases - antagonists & inhibitors; TOR Serine-Threonine Kinases - metabolism; Tunicamycin - pharmacology; Yeasts
• ISSN: 0032-0889; 1532-2548; 1532-2548
• DOI: 10.1104/pp.111.179861

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 TORCI and TORC2. Inactivation of TORCI 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.