Regulation of cystic fibrosis transmembrane regulator trafficking and protein expression by a Rho family small GTPase TC10.
The cystic fibrosis transmembrane conductance regulator (CFTR)-interacting protein, CFTR-associated ligand (CAL) down-regulates total and cell surface CFTR by targeting CFTR for degradation in the lysosome. Here, we report that a Rho family small GTPase TC10 interacts with CAL. This interaction specifically up-regulates CFTR protein expression. Co-expression of the constitutively active form, TC10Q75L, increases total and cell surface CFTR in a dose-dependent fashion. Moreover, co-expression of the dominant-negative mutant TC10T31N causes a dose-dependent reduction in mature CFTR. The effect of TC10 is independent of the level of CFTR expression, because a similar effect was observed in a stable cell line that expresses one-tenth of CFTR. Co-expression of TC10Q75L did not have a similar effect on the expression of plasma membrane proteins such as Frizzled-3 and Pr-cadherin or cytosolic proteins such as tubulin and green fluorescent protein. TC10Q75L also did not have a similar effect on the vesicular stomatitis virus glycoprotein. Co-expression of constitutively active and dominant-negative forms of Cdc42 or RhoA did not affect CFTR expression in a manner similar to TC10, indicating that the effect of TC10 is unique within the Rho family. Metabolic pulse-chase experiments show that TC10 did not affect CFTR maturation, suggesting that it exerts its effects on the mature CFTR. Importantly, TC10Q75L reverses CAL-mediated CFTR degradation, suggesting that TC10Q75L inhibits CAL-mediated degradation of CFTR. TC10Q75L does not operate by reducing CAL protein expression or its ability to form dimers or interact with CFTR. Interestingly, the expression of TC10Q75L causes a dramatic redistribution of CAL from the juxtanuclear region to the plasma membrane where the two molecules overlap. These data suggest that TC10 regulates both total and plasma membrane CFTR expression by interacting with CAL. The GTP-bound form of TC10 directs the trafficking of CFTR from the juxtanuclear region to the secretory pathway toward the plasma membrane, away from CAL-mediated degradation of CFTR in the lysosome.