A Genomic Analysis of Membrane Trafficking and Neurotransmitter Release in Drosophila

Intracellular compartments are maintained via an organized system of transport pathways that traffic lipids and proteins in vesicular organelles in a specific and regulated manner (Bennett and Scheller, 1993; Ferro-Novick and Jahn, 1994). The recent completion of the Drosophila genome (Adams et al., 2000; Rubin et al., 2000) allows us to analyze the z 14,000 genes that are encoded and begin to make evolutionary comparisons of mechanisms underlying membrane trafficking in metazoans. Models for intracellular trafficking have built upon the original SNARE hypothesis proposed by Söllner et al. (1993). In current models, the assembly and disassembly of a ternary complex composed of SNARE proteins is predicted to play a key role in vesicle–target membrane fusion. The neuronal SNARE complex, which is required for synaptic vesicle exocytosis at nerve terminals (Schulze et al., 1995; Deitcher et al., 1998; Littleton et al., 1998), is one of the bestcharacterized systems for intracellular fusion. The vesicle membrane v-SNARE, synaptobrevin, forms an SDS-resistant complex with the presynaptic membrane t-SNAREs, SNAP-25, and syntaxin 1. Within this complex, synaptobrevin and syntaxin each contribute one a -helix, while SNAP-25 contributes two a -helices (Sutton et al., 1998). These helices assemble to form a four-helix bundle which is thought to be characteristic of all cellular SNARE complexes throughout phylogeny. Assembly of the SNARE complex is required at a late post-docking stage in synaptic exocytosis (Littleton et al., 1998) and has been suggested to directly mediate bilayer membrane fusion (Weber et al., 1998). Disassembly of the SNARE complex by NSF and the SNAP adapter proteins is also required during neuronal vesicle cycling to recycle SNAREs for additional rounds of fusion (Littleton et al., 1998; Tolar and Pallanck, 1998). The regulation of SNARE assembly and disassembly, as well as the mechanisms for targeting vesicles to sites of SNARE fusion, are key processes that are likely conserved, but for which we know little about. An analysis of the proteins predicted by the Drosophila genome reveals a broad conservation of many trafficking proteins and several relatively large protein families involved in vesicle trafficking. Indeed, mammals, Drosophila , C . elegans , and yeast share a conserved core set of proteins involved in intracellular trafficking (Table I).