The overall goal of this proposal is to understand the biological processes involved in the protein folding, quality control and degradation of the membrane glycoprotein tyrosinase. Tyrosinase is a melanocyte-specific enzyme required for melanin synthesis. Mutations in tyrosinase is a melanocyte-specific enzyme required for melanin synthesis. Mutations in tyrosinase are the cause of tyrosinase-negative albinism. Many of these mutated forms contain amino acid substitution that have the potential to reduce the efficiency of proper folding and to cause retention in the endoplasmic reticulum (ER), the site of protein maturation for membrane glycoproteins. The key role of the ER in the regulation of tyrosine has been demonstrated in a melanotic melanoma cells in which ER-retention of wild type tyrosinase leads to subsequent degradation by the cytosolic ubiquitin-dependent proteasomal pathway. Understanding the mechanisms involved in tyrosinase biosynthesis and degradation will help us to address the following fundamental questions: Are the tyrosinase-mutants in albino melanocytes retained in the ER? What is the mechanism of retention? Can this retention be relieved to produce a correctly localized and functional protein? Can this retention by selectively implemented? The specific aims of this proposal are: 1) To analyze the protein maturation (folding, co- and post-translational modification, and chaperone binding) of wild type and mutant forms of tyrosinase in a cell-free maturation system. 2) To constitute the ER- retention/degradation pathway of tyrosinase in a cell-free system, identify and characterize the proteins involved in both the sorting and degradation processes. 3) To characterize the maturation, quality control and degradation of tyrosinase in normal and malignant melanocytes. These studies will provide insights into the maturation, quality control and degradation mechanisms of tyrosinase and the etiology of pigmentation-related diseases. In addition, a better understanding of the biosynthetic and degradation processes of the cell would assist in the design of therapeutic drugs targeted against genetic diseases caused by trafficking and ER maturation defects in general, and albinism and/or hypo- and hyperpigmentation in particular.
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