Melanosomes were isolated using OptiPrep® density gradient centrifugation (Fig 1). The RPE tissue was thawed on ice and suspended into hypotonic buffer containing inhibitors . Cells were disrupted with nitrogen cavitation at 450 psi with 15 min equilibration time. Whole cell lysate was centrifuged at 3000 g for 5 min and the supernatant (crude lysosomal fraction) was removed and pelleted at 20 000 g for 20 min. Crude melanosomal pellet was re-suspended into 50 mM Tris-HCl 150 mM KCl buffer (pH 7.40) and layered on top of discontinuous OptiPrep® gradient. Gradient tubes were centrifuged at 135 000 g for 1 h at + 4°C The melanosomal pellet was recovered and re-suspended into 50 mM Tris-HCl 150 mM KCl buffer (pH 7.40) and re-centrifuged in another OptiPrep® gradient. After the second gradient centrifugation, melanosomes were re-suspended into MES buffer (25 mM MES, 5 mM NaCl, 115 mM KCl, 1.3 mM MgSO4, pH 7.40) and residues of OptiPrep® were removed by two 10 000 g centrifugations. The total protein concentration of the purified melanosomal fraction was measured with the Bradford method