The presynaptic terminus is specialized for synaptic vesicle docking and release. Although much has been learned about neurotransmitter release, how the presynaptic terminus is organized is poorly understood. Our lab has used a presynaptic GFP marker (P
unc-25 -SNB-GFP) to study synapse formation in D neurons. In wild type animals, GFP fluorescent puncta that represent the presynaptic termini of VD and DD neurons are evenly distributed along ventral and dorsal cords. From a genetic screen, a number of mutants were isolated with abnormal morphology of the presynaptic termini, defining 12 genes named syd , for sy napse d efective (1). In
syd-3 mutants, the GFP puncta in the dorsal cord, corresponding to synapses of DD neurons, are sparse and irregular in size; in the ventral cord the puncta from VD neurons are slightly diffused. We mapped
syd-3 between
dpy-11 and
unc-42 on chromosome V, and cloned
syd-3 by germline transformation rescue.
syd-3 encodes a 3766 amino-acid residue protein with sequence similarity to human PAM (protein associated with Myc). PAM was identified based on interaction with Myc and is abundant in human brain tissue (2). Both SYD-3 and PAM have a guanine exchange factor (GEF) domain and a highly conserved C-terminal zinc finger cluster domain. A rescuing GFP tagged
syd-3 is expressed throughout the nervous system and localized to the presynaptic region. This synaptic localization is unchanged in
unc-104 mutants, suggesting that SYD-3 localization is independent of synaptic vesicle transport. Using a series of SYD-3-GFP translational fusions, we have identified a region in SYD-3 that may be required for synaptic localization of SYD-3.
syd-3 mutants alone do not exhibit obvious visible phenotypes. However,
syd-3;
syd-1 and
syd-3;
syd-2 double mutants are severely uncoordinated and display strong SNB--GFP defects.
syd-1 encodes a putative GAP protein (3), and
syd-2 encodes a Liprin (1). To identify other components in
syd-3 signaling, we performed a suppressor screen on
syd-3;
syd-1 double mutants and isolated a number of suppressors for
syd-3 . We will report molecular analysis of
syd-3 and characterization of the suppressors. 1. Zhen and Jin (1999) International meeting abstract 2. Guo Q. et al. PNAS 95: 9172 (1998) 3. Hallam and Jin (1999) International meeting abstract