THE SYNAPTIC PROTEIN SYNTAXIN-1 IS REQUIRED FOR CELLULARIZATION OF DROSOPHILA EMBRYOS

Syntaxins are membrane proteins involved in vesicle trafficking and ar e required for the release of neurotransmitter at nerve terminals. The presence of syntaxins on target membranes has been hypothesized to co nfer specificity to targeting and fusion via interactions with complem entary vesicle-associated proteins, the synaptobrevins or VAMPS. We ha ve mutagenized syntaxin1 in Drosophila and have found that it links th e mechanism of synaptic transmission to a distinct cell biological pro cess: the cellularization of early embryos. This specialized form of c ell division separates the 6,000 nuclei of the syncytial blastoderm in to separate cells through the invagination of the surface membrane of the embryo. During this process, syntaxin1 protein is present on the n ewly forming lateral cell surfaces and invaginating cleavage furrows. This protein is derived both from maternal deposition of mRNA and prot ein and from early zygotic transcription. To analyze syntaxin1's role in early development, female germ line mosaics mutant for syntaxin1 ex pression were generated by mitotic recombination to reduce the materna l contribution. Visualizing the actin cytoskeleton and glycosylated su rface proteins reveals that embryos with insufficient syntaxin1 have l arge acellular patches. The patches do not appear until cellularizatio n begins, and the process fails entirely within these regions. These r esults provide genetic evidence that membrane trafficking is required for the cellularization of the syncytial blastoderm. We propose that t he invagination of the surface membrane proceeds by the fusion of intr acellular membrane vesicles with the surface. This reaction uses the s ame syntaxin1 protein as is required for neurotransmitter secretion at synapses. Thus, a single syntaxin can participate in trafficking step s that are functionally as distinct as synaptic transmission and cell division.