Figure 3.

Membrane-associated transporter protein (MATP), a putative sucrose/H⁺ symporter, is present in the melanosome. MATP regulates melanosomal pH for copper binding in the active site of TYR. In the oculocutaneous albinism type 4 (OCA4) melanosome, mutations in MATP cause a loss of function, and the inner melanosomal pH becomes acidic, following the loss of copper ions from the active form of TYR. As a result, melanin production is downregulated. The arrows indicate the direction of substrates such as proton and copper.