Synaptic localization and restricted diffusion of a Drosophila neuronal synaptobrevin--green fluorescent protein chimera in vivo.

Fluorescent markers for subcellular compartments in Drosophila neurons should allow one to combine genetic mutant analysis with visualization of subcellular structures in vivo. Here we describe an analysis of two markers which may be used to observe different compartments of live Drosophila synapses. Soluble jellyfish green fluorescent protein (GFP) expressed at high levels in neurons diffuses freely in the neuronal cytosol as evidenced by confocal microscopy and fluorescence recovery from photobleaching experiments. Thus, the distribution pattern of soluble GFP in motor axons and larval motor terminals indicates the expected distribution for diffusible presynaptic molecules. In contrast to GFP, a neurally expressed neuronal synaptobrevin-GFP chimera (n-syb GFP) is transported down axons and specifically localized to nerve terminals. We demonstrate that n-syb GFP labels synaptic-vesicle membrane at larval motor terminals by documenting its restriction to presynaptic varicosities, its colocalization with synaptic vesicle antigens, and its redistribution in Drosophila shits1 mutant nerve terminals transiently depleted of synaptic vesicles. Surprisingly, n-syb GFP expressed in muscle is concentrated at the subsynaptic reticulum (SSR), postsynaptic infoldings of muscle plasma membrane. We suggest, using different membrane markers, that this apparent postsynaptic enrichment simply reflects a concentration of plasma membrane in the SSR, rather than a selective targeting of n-syb GFP to postsynaptic sites. Utilities and implications of these studies are demonstrated or discussed.