Altered expression of a novel adaptin leads to defective pigment granule biogenesis in the Drosophila eye color mutant garnet

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Abstract
Drosophila eye pigmentation defects have thus far been attributed to mutations in genes encoding enzymes required for biosynthesis of pigments and to ABC‐type membrane transporters for pigments or their precursors. We report here that a defect in a gene encoding a putative coat adaptor protein leads to the eye color defect of garnet mutants. We first identified a human cDNA encoding δ‐adaptin, a structural homolog of the α‐ and γ‐adaptin subunits of the clathrin coat adaptors AP‐1 and AP‐2, respectively. Biochemical analyses demonstrated that δ‐adaptin is a component of the adaptor‐like complex AP‐3 in human cells. We then isolated a full‐length cDNA encoding the Drosophila ortholog of δ‐adaptin and found that transcripts specified by this cDNA are altered in garnet mutant flies. Examination by light and electron microscopy indicated that these mutant flies have reduced numbers of eye pigment granules, which correlates with decreased levels of both pteridine (red) and ommachrome (brown) pigments. Thus, the eye pigmentation defect in the Drosophila garnet mutant may be attributed to compromised function of a coat protein involved in intracellular transport processes required for biogenesis or function of pigment granules.