Solute carrier family 45 member 2 encodes the melanosomal membrane protein, membrane-associated transporter protein (MATP), of unknown function, that is required for normal melanin synthesis. The present study analyzed the effects of two human MATP mutations, D93N, which causes oculocutaneous albinism 4 (OCA4), and L374F, which is correlated with light pigmentation in European populations. Corresponding mutations were produced in the related and well-characterized sucrose transporter from rice, OsSUT1, and transport activity was measured by heterologous expression in Xenopus laevis oocytes, in addition to 14C-sucrose uptake in yeast. The mutation corresponding to D93N resulted in a complete loss of transport activity. The mutation corresponding to L374F resulted in a 90% decrease in transport activity, although the substrate affinity was unaffected. The results indicated that the D93N mutation causes OCA4 as a result of loss of MATP transport activity, and that the F374 allele confers significantly lower transport activity than L374.