Oculocutaneous albinism type 1TS is caused by mutations that render the melanocyte-specific enzyme tyrosinase temperature-sensitive (ts); the enzyme is inactive in cells grown at 37 degrees C but displays full activity in cells grown at 31 degrees C. To distinguish whether the ts phenotype of the common R402Q variant of human tyrosinase is due to altered enzymatic activity or to misfolding and a defect in intracellular trafficking, we analyzed its localization and processing in transiently transfected HeLa cells. R402Q tyrosinase accumulates in the endoplasmic reticulum (ER) at 37 degrees C but 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 tyrosinase, is not dependent on tethering to the membrane, and is irreversible. Finally, a wild-type allelic form of tyrosinase is partially ts in transiently transfected HeLa cells. The data show that human tyrosinase expressed in non-melanogenic cells folds and exits the ER inefficiently and that R402Q tyrosinase exaggerates this defect, resulting in a failure to exit the ER at physiologic temperatures.