Endoplasmic reticulum retention is a common defect associated with tyrosinase-negative albinism

Tyrosinase is a melanocyte-specific enzyme critical for the synthesis of me lanin, a process normally restricted to a post-Golgi compartment termed the melanosome. Loss-of-function mutations in tyrosinase are the cause of ocul ocutaneous albinism, demonstrating the importance of the enzyme in pigmenta tion. In the present study, we explored the possibility that trafficking of albino tyrosinase from the endoplasmic reticulum (ER) to the Golgi apparat us and beyond is disrupted. Toward this end, we analyzed the common albino mouse mutation Tyr(C85S), the frequent human albino substitution TYR(T373K) , and the temperature-sensitive tyrosinase TYR(R402Q)/Tyr(H402A) found in h umans and mice, respectively. Intracellular localization was monitored in a lbino melanocytes carrying the native mutation, as well as in melanocytes e ctopically expressing green fluorescent protein-tagged tyrosinase. Enzymati c characterization of complex glycans and immunofluorescence colocalization with organelle-specific resident proteins established that all four mutati ons produced defective proteins that were retained in the ER. NR(R402Q)/Tyr (H402A) Golgi processing and transport to melanosomes were promoted at the permissive temperature of 32 degrees C, but not at the nonpermissive 37 deg rees C temperature. Furthermore, evidence of protein misfolding was demonst rated by the prolonged association of tyrosinase mutants with calnexin and calreticulin, known ER chaperones that play a key role in the quality-contr ol processes of the secretory pathway. From these results we concluded that albinism, at least in part, is an ER retention disease.