Presynaptic terminals are highly specialized subcellular structures to facilitate neurotransmitter release. In a presynaptic terminal, synaptic vesicles are docked around the electron dense presynaptic specializations. The periactive zone surrounds the vesicle clustering and active zone region. In
rpm-1 mutants, presynaptic terminals at the GABAergic neuromuscular junctions are disorganized and the mechanosensory neurons synapses are not stabilized (1, 2). RPM-1 is a member of the PHR protein family that includes Drosophila Highwire, Mammalian Phr and Pam, and is localized to the periactive zone. RPM-1 and its functional partner FSN-1 are components of an SCF-like E3 ubiquitin ligase (3). Through the study of genetic suppressor mutations of
rpm-1, we have shown that RPM-1 negatively regulates a
p38 MAP kinase pathway that is composed of the MAPKKK DLK-1, the MAPKK MKK-4, and the
p38 MAP kinase PMK-3 (4). To explore additional signaling components involved in the RPM-1 and MAPK pathway, we analyzed novel suppressor mutations and found that loss-of-function mutations in
uev-3 suppress Rpm-1 phenotypes in a manner similar to mutations in the MAP kinases. UEV-3 is a ubiquitin conjugating enzyme E2 variant. The expression of
uev-3 in presynaptic neurons rescued the suppression of
rpm-1(lf) by
uev-3(lf), whereas the expression of
uev-3 in postsynaptic muscles did not. Furthermore, results from genetic epistasis analysis are consistent with a conclusion that UEV-3 functions in the RPM-1-DLK-1-
p38 MAP kinase pathway and acts downstream of MKK-4. UEV-3::GFP is localized in nuclei and cytoplasm of motor neurons, similar to that of PMK-3::GFP. By a yeast two-hybrid assay we found UEV-3 physically interacted with PMK-3 but not with DLK-1 and MKK-4. Our results suggest that UEV-3 may promote PMK-3 function by acting as a scaffolding protein or as a chaperone in synaptogenesis. 1) Zhen, M. et al. Neuron, 26: 331-343, 2000 2) Schafer et al. Neuron, 26: 345-356, 2000 3) Liao, E. H. et al. Nature, 430: 345-350, 2004 4) Nakata, K. et al. Cell, 120: 407-420, 2005.