A common temperature-sensitive allelic form of human tyrosinase is retained in the endoplasmic reticulum at the nonpermissive temperature

Oculocutaneous albinism type 1TS is caused by mutations that render the mel anocyte-specific enzyme tyrosinase temperature-sensitive (ts); the enzyme i s inactive in cells grown at 37 degrees C but displays full activity in cel ls grown at 31 degrees C. To distinguish whether the ts phenotype of the co mmon R402Q variant of human tyrosinase is due to altered enzymatic activity or to misfolding and a defect in intracellular trafficking, we analyzed it s localization and processing in transiently transfected HeLa cells. R402Q tyrosinase accumulates in the endoplasmic reticulum (ER) at 37 degrees C bu t exits the ER and accumulates in endosomal structures in cells grown at 31 degrees C. The inability of the R402Q variant to exit the ER is confirmed by the failure to acquire endoglycosidase H resistance at 37 degrees C and cannot be accounted for solely by enhanced proteasome-mediated degradation. ER retention at 37 degrees C is mediated by the lumenal domain of R402Q ty rosinase, is not dependent on tethering to the membrane, and is irreversibl e. Finally, a wild-type allelic form of tyrosinase is partially ts in trans iently transfected HeLa cells. The data show that human tyrosinase expresse d in non-melanogenic cells folds and exits the ER inefficiently and that R4 02Q tyrosinase exaggerates this defect, resulting in a failure to exit the ER at physiologic temperatures.