unc-51 ,
unc-14 and
unc-33 genes are required for axonal elongation, fasciculation and guidance of many neurons. Previously, we reported that
unc-51 encodes a novel serine / threonine kinase and that UNC-51 directly interacts with a novel protein UNC-14. On the other hand, we isolated CRMP-62 (colapsin response mediator protein) and found that the protein shares homology with UNC-33. These genes are expressed in many neurons, suggesting that these molecules regulate axon guidance in neurons. However, their functions are largely unknown. In order to analyze their functions, we newly identified interacting proteins with UNC-51, UNC-14 or UNC-33 by using a yeast two hybrid system. In this screening, UNC-73 (a Trio homolog) and VAB-8 (a kinesin like protein) were identified as UNC-51 interacting proteins. A C. elegans homolog of filamin 1 (FLN1) was identified as a UNC-33 interacting protein. UNC-73 is a GEF (guanine nucleotide exchange factor) and is required for axon guidance and cell migrations. The GEF domain activates Rac GTPase and stimulates actin polymerization. We examined genetic interaction between
unc-51 and
unc-73 on circumferential axonal defects of GABAergic DD/VD neurons. We found that
e936 (a hypomorphic allele of
unc-73 ) strongly enhanced the defects of
e369 (a strong allele of
unc-51 ), suggesting that these molecules function at the same signaling pathway, probably for regulation of actin polymerization. The defects of
e369;
e936 double mutants were severer than
gm40 (a null allele of
unc-73 ), suggesting that UNC-51 regulates other pathways as well. In
unc-51 mutants, abnormally large varicosities are observed in presynaptic axons of ASI (Crump et al., Neuron, 29, 115-129, 2001). We examined presynaptic morphology of the ASI in
unc-14(
e57) and
unc-73(
e936) mutants, and found that, at low penetrance, similar large varicosities were also observed in
unc-14(
e57) mutants. We think that UNC-14, in corporation with UNC-51, regulates presynaptic formation as well. We did not detect such large varicosities in
unc-73(
e936) mutants. UNC-73 may not cooperate with UNC-51 on presynaptic formation. Analysises of other interacting proteins are in progress. We will discuss possible mechanisms and functions of these molecules on regulation of axon guidance and presynaptic formation.