Several cells in C. elegans traverse long range migrations during development. The
unc-53 gene is required for the guidance and extension of a subset of cells along the anterior-posterior axis of the worm, including the mechanosensory neurons, sex muscles and the excretory canals. UNC-53 binds F-actin and SEM-5/GRB2 in vitro, implicating UNC-53 in both signal transduction and actin cytoskeleton dynamics. The
unc-53 locus is large, containing 23 exons spanning 31kb of genomic DNA. In addition to the presence of multiple promoters,
unc-53 transcripts are subject to alternative splicing, suggesting the existence of multiple protein isoforms in vivo, several of which are truncated at the N-terminus. (Stringham, Pujol et al., 2002,Development 129: 3367-3379). UNC-53 contains a single CH domain at its N-terminus, a motif that is found in a variety of cytoskeletal and signal transduction molecules, and which is implicated in the regulation of cell shape dynamics. Immunofluorescence of adult hermaphrodites with antisera against this domain stains the excretory canals, coelomocytes and some neurons suggesting that the full length UNC-53 isoform inclusive of the CH domain is required in these cells even after completion of outgrowth. Using the CH domain as bait and a C. elegans cDNA library as prey in a yeast two hybrid screen, we have identified 25 strong candidate interactors. Five of the cDNAs correspond to the C. elegans genomic locus B0336.6, which encodes a polypeptide of 469 amino acids and contains an SH3 domain at the C-terminus suggesting a role in signal transduction. The SH3 domain of the Drosophila homologue, ABI, binds to the polyproline motif of the Abelson tyrosine kinase (Juang et al., 1999, Oncogene 18:5138-5147) and the yeast homologue, Ysc84p, is thought to couple the actin cytoskeleton to the endocytic machinery (Dewar et al, 2002, MBC 13:3646-3661). Two candidate cDNAs encode C. elegans REF (2) P like protein that contains a zinc finger motif of the ZZ type found in the cytoskeletal protein dystrophin, and a ubiquitin associated domain at the C-terminus. The yeast homologue, verprolin, is an actin binding protein that is thought to modulate polarization of the actin cytoskeleton (Vaduva et al., 1997, JCB 139:1821-1833). Interestingly, neither of these candidate UNC-53 interactors were discovered in previous genetic screens aimed at identifying genes involved in cell migration and outgrowth. (Supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada to E.S.)