Neuronal synapses are asymmetric structures consisting of pre and post-synaptic cell terminals in direct apposition to each other. Presynaptic terminals can be visualized using a vesicle associated synaptobrevin::GFP (SNB-1::GFP) marker [1,2]. Previous screens using this marker have identified
rpm-1 (regulator of presynaptic morphology) as a novel regulator of synaptic development [3]. Some GABAergic neuromuscular junctions in
rpm-1 mutants have abnormal clusters of SNB-1::GFP puncta and an excessive number of active zones. This suggests that
rpm-1 is a negative regulator of presynaptic differentiation. RPM-1 is a large presynaptic protein with a putative guanine nucleotide exchange factor and a RING zinc finger domain. RING zinc finger domains have been identified in protein components of the E3 ubiquitin ligase complex [4]. The presence of this domain in
rpm-1 suggests it may have E3 ubiquitin ligase activity. Here we report the identification of a new gene
fsn-1 (F-box protein at the synapse) that functions in the
rpm-1 pathway.
fsn-1 null animals display identical phenotypes as
rpm-1. They have irregular clustering of SNB-1::GFP and no obvious behavioral defects. Moreover,
fsn-1;
rpm-1 double mutants are identical to either mutant alone.
fsn-1 encodes a protein with an F-box motif and a SPRY domain.
fsn-1 is expressed exclusively in the nervous system. Expression of
fsn-1 in pre-synaptic neurons is sufficient to fully rescue the mutant phenotype, suggesting that
fsn-1 function is both necessary and sufficient in presynaptic cells. The majority of F-box proteins act as substrate specificity factors for SCF (Skp1, Cullin, F-box) E3 ubiquitin ligase complexes. The SCF type E3 ubiquitin ligase complex also includes a RING finger domain protein [5, 6, 7]. Based on the evidence that
rpm-1 and
fsn-1 function in the same pathway, we propose a model that FSN-1 and RPM-1 form an SCF like E3 ubiquitin ligase complex. This complex could control ubiquitin mediated proteolysis at presynaptic termini. The turnover of unknown protein factors is essential for restricting the presynaptic differentiation in at least some neuromuscular junctions.1.Nonet, M.L. J Neurosci Methods, 1999. 89(1): p. 33-40. 2.Jin, Y., et al. J Neurosci, 1999. 19(2): p. 539-48. 3.Zhen, M., et al. Neuron, 2000. 26(2): p. 331-43. 4.Kamura, T, et al. Science 1999. 284: p.657-661 5.Patton, EE, et al. Trends Genetics, 1998. 14: p. 236-243 6.Skowyra et al. (1997) Cell 91: p. 201-19 7.Feldman et al. (1997) Cell 91: p. 221-30