Abstract
Peptidyl-prolyl cis/trans isomerases (PPIs) catalyze cis/trans isomerization of peptide bonds preceding proline residues. The involvement of PPI family members in protein refolding has been established in test tube experiments. Surprisingly, however, no data is available on the involvement of endoplasmic reticulum (ER)-resident members of the PPI family in protein folding, quality control or disposal in the living cell. Here we report that the immunosuppressive drug cyclosporine A (CsA) selectively inhibits the degradation of a subset of misfolded proteins generated in the ER. We identify cyclophilin B (CyPB) as the ER-resident target of CsA that catalytically enhances disposal from the ER of ERAD-LS substrates containing cis proline residues. Our manuscript presents the first evidence for enzymatic involvement of a PPI in protein quality control in the ER of living cells.
Highlights
Formation and reduction of covalent bonds between cysteine side chains and cis/trans isomerization of peptide bonds preceding proline residues are rate-determining steps for the attainment of the native and functional 3D structure of polypeptides synthesized in the endoplasmic reticulum (ER)
It is impossible to establish if, and which one of the peptidyl bonds preceding these proline residues is converted from the trans to the cis configuration during the short retention of these folding-defective polypeptides in the ER lumen
To assess whether prolyl isomerases might facilitate disposal of BACE457 and BACE457D from the mammalian ER, we exposed cells transiently transfected for expression of either one of the two model substrates to cyclosporine A (CsA), a selective inhibitor of immunophilin members of the Peptidyl-prolyl cis/trans isomerases (PPIs) family [24]
Summary
Formation and reduction of covalent bonds between cysteine side chains and cis/trans isomerization of peptide bonds preceding proline residues are rate-determining steps for the attainment of the native and functional 3D structure of polypeptides synthesized in the ER. We report that the immunosuppressive drug CsA, a specific inhibitor of the cyclophilin family of PPIs, selectively delays the degradation of the ERAD-LS substrate BACE457D leaving unaffected disposal from the ER of the same polypeptide when tethered to the ER membrane (the ERAD-LM protein BACE457). The presence of peptidyl-prolyl bonds in the cis conformation renders disposal of ERAD-LS substrates sensitive to CsA and dependent on CyPB intervention.
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