Murine and human b locus pigmentation genes encode a glycoprotein (gp75) with catalase activity

Area of Science:

• Biochemistry
• Genetics
• Melanogenesis Research

Background:

• Melanogenesis, the process of pigment production, is primarily regulated by tyrosinase.
• Defects in tyrosinase cause albinism, but other pigmentation determinants remain unclear.
• The murine brown (b) locus mutation results in altered coat color (brown or hypopigmented).

Purpose of the Study:

• To elucidate the function of the murine brown (b) locus in pigmentation.
• To determine the enzymatic activity of the protein encoded by the b locus.
• To understand the molecular basis of coat color variation in mice.

Main Methods:

• Biochemical assays to determine enzyme activity.
• Genetic analysis of murine coat color mutants.
• Protein characterization, including mutation analysis and stability studies.

Main Results:

• The b locus encodes a glycoprotein with catalase activity (catalase B), not tyrosinase.
• Catalase B is identified as the melanosomal glycoprotein gp75.
• Mutations in the b locus affect the heme-associated domain, leading to protein degradation and altered pigmentation.

Conclusions:

• Pigmentation is controlled by both tyrosinase and a hydroperoxidase (catalase B).
• Hydrogen peroxide, a byproduct of melanogenesis, can degrade melanin precursors and melanin.
• The b mutation's impact on catalase B stability explains its role in coat color determination.