The Caenorhabditis elegans unc-13 mutant is a member of a class of mutants that have un-coordinated movement. Mutations of the unc-13 gene cause diverse defects in C. elegans, including abnormal neuronal connections and modified synaptic transmission in the nervous system. unc-13 cDNA encodes a protein (UNC-13) of 1734 amino acid residues with a predicted molecular mass of 198 kDa and sequence identity to the C1/C2 regions but not to the catalytic domain of the ubiquitously expressed protein kinase C family [Maruyama & Brenner (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 5729-5733]. To characterize the phorbol ester binding site of the UNC-13 protein, cDNA encoding the C1/C2-like regions (amino acid residues 546-940) was expressed in Escherichia coli and the 43 kDa recombinant protein was purified. Phorbol ester binding to the 43 kDa protein was zinc- and phospholipid-dependent, stereospecific and of high affinity (Kd 67 nM). UNC-13 specific antisera detected a protein of approx. 190 kDa in wild-type (N2) but not in mutant (e1019) C. elegans cell extracts. We conclude that UNC-13 represents a novel class of phorbol ester receptor.