The V(0) complex forms the proteolipid pore of an ATPase that acidifies vesicles. In addition, an independent function in membrane fusion has been proposed largely based on yeast vacuolar fusion experiments. We have isolated mutations in the largest V(0) component vha100-1 in flies in an unbiased genetic screen for synaptic malfunction. The protein is only required in neurons, colocalizes with markers for synaptic vesicles as well as active zones, and interacts with t-SNAREs. Loss of vha100-1 leads to vesicle accumulation in synaptic terminals, suggesting a deficit in release. The amplitude of spontaneous release events and release with hypertonic stimulation indicate normal levels of neurotransmitter loading, yet mutant embryos display severe defects in evoked synaptic transmission and FM1-43 uptake. Our data suggest that Vha100-1 functions downstream of SNAREs in synaptic vesicle fusion.

Ionotropic glutamate receptors (iGluRs) bind agonists in a domain that has been crystallized and shown to have a bilobed structure. Eukaryotic iGluRs also possess a second extracellular N-terminal domain related to the bacterial periplasmic binding protein LIVBP. In NMDA receptors, the high-affinity Zn inhibition is eliminated by mutations in the LIVBP-like domain of the NR2A subunit. Using LIVBP structure, we have modeled this domain as two lobes connected by a hinge and show that six residues controlling Zn inhibition form two clusters facing each other across a central cleft. Upon Zn binding the two lobes close tightly around the divalent cation. Thus, the extracellular region of NR2A consists of a tandem of Venus flytrap domains, one binding the agonist and the other a modulatory ligand. Such a functional organization may apply to other eukaryotic iGluRs.

The first basalar muscle (b1) is one of 17 small muscles in flies that control changes in wing stroke kinematics during steering maneuvers. The b1 is unique, however, in that it fires a single phase-locked spike during each wingbeat cycle. The phaselocked firing of the b1's motor neuron (mnb1) is thought to result from wingbeat-synchronous mechanosensory input, such as that originating from the campaniform sensilla at the base of the halteres. Halteres are sophisticated equilibrium organs of flies that function to detect angular rotations of the body during flight. We have developed a new preparation to determine whether the campaniform sensilla at the base of the halteres are responsible for the phasic activity of b1. Using intracellular recording and mechanical stimulation, we have found one identified haltere campaniform field (dF2) that provides strong synaptic input to the mnb1. This haltere to mnb1 connection consists of a fast and a slow component. The fast component is monosynaptic, mediated by an electrical synapse, and thus can follow haltere stimulation at high frequencies. The slow component is possibly polysynaptic, mediated by a chemical synapse, and fatigues at high stimulus frequencies. Thus, the fast monosynaptic electrical pathway between haltere afferents and mnb1 may be responsible in part for the phase-locked firing of b1 during flight.

Acetylcholine is the major excitatory neurotransmitter in the central nervous system of insects. Mutant analysis of the Dα7 nicotinic acetylcholine receptor (nAChR) ofDrosophila shows that it is required for the giant fiber-mediated escape behavior. The Dα7 protein is enriched in the dendrites of the giant fiber, and electrophysiological analysis of the giant fiber circuit showed that sensory input to the giant fiber is disrupted, as is transmission at an identified cholinergic synapse between the peripherally synapsing interneuron and the dorsal lateral muscle motor neuron. Moreover, we found thatgfA1, a mutation identified in a screen for giant fiber defects more than twenty years ago, is an allele ofDα7. Therefore, a combination of behavioral, electrophysiological, anatomical, and genetic data indicate an essential role for the Dα7 nAChR in giant fiber-mediated escape inDrosophila.