ALPHA-ADAPTIN, A MARKER FOR ENDOCYTOSIS, IS EXPRESSED IN COMPLEX PATTERNS DURING DROSOPHILA DEVELOPMENT

A Drosophila cDNA encoding a structural homologue of the mammalian coa ted vesicle component alpha-adaptin (AP2 adaptor complex) has been clo ned and sequenced. The mammalian and invertebrate sequences are highly conserved, especially within the amino terminal region, a domain that mediates interactions with other components within the AP2 complex an d with specific receptor tails. Mammalian alpha-adaptins are encoded b y two genes; however, Drosophila alpha-adaptin has a single gene locus , within polytene bands 21C2-C3 on the left arm of chromosome 2, close ly adjacent to the paired homeobox gene aristaless. There seem to be a t least two Drosophila alpha-adaptin transcripts expressed, plausibly by alternative splicing. One of the transcripts is more abundant durin g early embryogenesis and may be of maternal origin. We have studied t he distribution of the alpha-adaptin protein throughout embryogenesis and at the neuromuscular junction of the third instar larva. During ce llularization of the blastoderm embryo, the protein is seen between an d ahead of the elongating nuclei, and then redistributes to the cell s urface during gastrulation. These observations suggest a role for endo cytosis in cellularization and are consistent with the finding that dy namin (the shibire gene product), another component of the endocytic m echanism, is required for cellularization. At later stages of embryoge nesis, alpha-adaptin is expressed in complex and dynamic patterns. It is strongly induced in elements of the central and peripheral nervous system (e.g., in neuroblasts, the presumptive stomatogastric nervous s ystem, and the lateral chordotonal sense organs), in the Garland cells , the adult midgut precursors, the antenno-maxillary complex, the endo derm, the fat bodies, and the visceral mesoderm. In the larva, alpha-a daptin is localized at the plasma membrane in the synaptic boutons of the neuromuscular junctions. The cells expressing high levels of alpha -adaptin are known or expected to support high levels of endocytosis; thus, this coated vesicle protein seems to be an excellent marker for endocytic activity. The expression patterns of dynamin, detected in th e embryo by in situ hybridization methods, are very similar to those r eported here for alpha-adaptin reflecting the likely coordinated expre ssion of endocytic components. Taken together with previous evidence, our results suggest that endosomal vesicle trafficking, membrane recyc ling, and the regulation of endocytosis play critical roles in the wid e range of developmental processes.