Defective expression of the μ3 subunit of the AP-3 adaptor complex in the Drosophila pigmentation mutant carmine

The adaptor protein (AP) complexes AP-1, AP-2, and AP-3 mediate coated vesicle formation and sorting of integral membrane proteins in the endocytic and late exocytic pathways in mammalian cells. A search of the Drosophila melanogaster expressed sequence tag (EST) database identified orthologs of family members mammalian medium (II) chain families mu 1, mu 2, and mu 3, of the corresponding AP complexes, and delta-COP, the analogous component of the coatomer (COPI) complex. The Drosophila orthologs exhibit a high degree of sequence identity to mammalian medium chain and delta-COP proteins. Northern analysis demonstrated that medium chain and delta-COP mRNAs are expressed uniformly throughout fly development. Medium chain and delta-COP genes were cytologically mapped and the mu 3 gene was found to localize to a region containing the pigmentation locus carmine (cm). Analysis of genomic DNA of the cml mutant allele indicated the presence of a large insertion in the coding region of the mu 3 gene and Northern analysis revealed no detectable mu 3 mRNA. Light microscopy of the cm(1) mutant showed a reduction in primary, secondary, and tertiary pigment granules in the adult eye. These findings provide evidence of a role for mu 3 in the sorting processes required for pigment granule biogenesis in Drosophila.