Sub-cellular localisation of the White/Scarlet ABC transporter to pigment granule membranes within the compound eye of Drosophila melanogaster

The white, scarlet, and brown genes of Drosophila melanogaster encode ABC transporters involved with the uptake and storage of metabolic precursors to the red and brown eye colour pigments. It has generally been assumed that these proteins are localised in the plasma membrane and transport precursor molecules from the heamolymph into the eye pigment cells. However, the immuno-electron microscopy experiments in this study reveal that the White and Scarlet proteins are located in the membranes of pigment granules within pigment cells and retinula cells of the compound eye. No evidence of their presence in the plasma membrane was observed. This result suggests that, rather than transporting tryptophan into the cell across the plasma membrane, the White/Scarlet complex transports a metabolic intermediate (such as 3-hydroxy kynurenine) from the cytoplasm into the pigment granules. Other functional implications of this new finding are discussed.