The Activity and Stability of p56Lck and TCR Signaling Do Not Depend on the Co-Chaperone Cdc37

Lymphocyte-specific protein tyrosine kinase (Lck) is a pivotal tyrosine kinase involved in T cell receptor (TCR) signaling. Because of its importance, the activity of Lck is regulated at different levels including phosphorylation of tyrosine residues, protein–protein interactions, and localization. It has been proposed that the co-chaperone Cdc37, which assists the chaperone heat shock protein 90 (Hsp90) in the folding of client proteins, is also involved in the regulation of the activity/stability of Lck. Nevertheless, the available experimental data do not clearly support this conclusion. Thus, we assessed whether or not Cdc37 regulates Lck. We performed experiments in which the expression of Cdc37 was either augmented or suppressed in Jurkat T cells. The results of our experiments indicated that neither the overexpression nor the suppression of Cdc37 affected Lck stability and activity. Moreover, TCR signaling proceeded normally in T cells in which Cdc37 expression was either augmented or suppressed. Finally, we demonstrated that also under stress conditions Cdc37 was dispensable for the regulation of Lck activity/stability. In conclusion, our data do not support the idea that Lck is a Cdc37 client.     

Regulation of ClC-2 Chloride Channel Proteostasis by Molecular Chaperones: Correction of Leukodystrophy-Associated Defect

The ClC-2 channel plays a critical role in maintaining ion homeostasis in the brain and the testis. Loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the white matter disease leukodystrophy. Clcn2-deficient mice display neuronal myelin vacuolation and testicular degeneration. Leukodystrophy-causing ClC-2 mutant channels are associated with anomalous proteostasis manifesting enhanced endoplasmic reticulum (ER)-associated degradation. The molecular nature of the ER quality control system for ClC-2 protein remains elusive. In mouse testicular tissues and Leydig cells, we demonstrated that endogenous ClC-2 co-existed in the same protein complex with the molecular chaperones heat shock protein 90β (Hsp90β) and heat shock cognate protein (Hsc70), as well as the associated co-chaperones Hsp70/Hsp90 organizing protein (HOP), activator of Hsp90 ATPase homolog 1 (Aha1), and FK506-binding protein 8 (FKBP8). Further biochemical analyses revealed that the Hsp90β-Hsc70 chaperone/co-chaperone system promoted mouse and human ClC-2 protein biogenesis. FKBP8 additionally facilitated membrane trafficking of ClC-2 channels. Interestingly, treatment with the Hsp90-targeting small molecule 17-allylamino-17-demethoxygeldanamycin (17-AAG) substantially boosted ClC-2 protein expression. Also, 17-AAG effectively increased both total and cell surface protein levels of leukodystrophy-causing loss-of-function ClC-2 mutant channels. Our findings highlight the therapeutic potential of 17-AAG in correcting anomalous ClC-2 proteostasis associated with leukodystrophy.