Synapses are asymmetric junction structures specialized for signal transduction from neurons to target cells. Active zone in presynaptic terminal has a characteristic electron-dense structure surrounded by a cluster of synaptic vesicles and promotes neurotransmitters release. A unique set of proteins is accumulated in active zone and functions in synaptic vesicle docking, priming, and fusion, synaptic target recognition and adhesion, or organization of active zone structure. In mutants lacking
syd-2, electron-dense structures are abnormal in size and appearance, synaptic vesicles are fewer and diffusely localized, and active zone proteins such as RIM and ELKS are dispersed. SYD-2 is a member of the conserved Liprin-<font face=symbol>a</font> protein family, which contains coiled-coil domains at the N-terminus and SAM domains at the C-terminus. SYD-2 is localized to active zones. We have recently reported that a gain-of-function allele of
syd-2,
syd-2(
ju487gf), suppresses the HSN synapse defects caused by loss of function in
syd-1 (Dai et al. 2006). HSN neurons form synapses to the vulval muscles and VC neurons.
syd-2(lf) and
syd-1(lf) mutants are Egl, due to failure of accumulating synaptic components at the vulva.
syd-2(
ju487gf);
syd-1(lf) double mutants are not Egl, indicating that
syd-2(
ju487gf) bypasses the requirement of
syd-1. This suppression effect of
syd-2(
ju487gf) requires
elks-1. Overexpression of wild type
syd-2 as well as
syd-2(
ju487gf) both suppresses
syd-1(lf) phenotype in HSN, whereas overexpression of
elks-1 does not. These results suggest that
syd-2(
ju487gf) mutation elevates the activity of
syd-2 to promote synaptic formation. Thus, the activity level of SYD-2 is the critical limiting factor. One of the important questions is how the
syd-2 activity can be elevated in
syd-2(
ju487gf).
ju487gf is a missense mutation altering Arg184 residue to Cys at the N-terminal highly conserved coiled-coil structure called LH1 domain. SYD-2 R184C binds and recruits more ELKS-1 protein to synaptic location. However, the LH1 domain is not in the previously reported binding region of SYD-2/Liprin-<font face=symbol>a</font> for ELKS. One possibility is that R184C affects overall SYD-2 protein conformation to enhance the interaction with ELKS-1. Alternatively, R184C may change the status of SYD-2 homophilic interaction mediated through N-terminal coiled-coil regions. We are testing these possibilities biochemically using recombinant proteins of SYD-2. We have also executed transgenic rescue experiments to identify the region of SYD-2 that is sufficient for the suppression of
syd-1(lf) phenotype in HSN. These analyses will give us a mechanistic insight into SYD-2 activation, a key step in the organization of active zone architecture.