Protein interaction surface of the POU transcription factor UNC-86 selectively used in touch neurons

The Caenorhabditis elegans POU protein UNC-86 specifies the HSN motor neuro ns, which are required for egg-laying, and six mechanosensory neurons. To i nvestigate how UNC-86 controls neuronal specification, we characterized two UNC-86 mutants that do not respond to touch but show wild-type egg-laying behavior. Residues P145 and L195, which are altered by these mutations, are located in the POU-specific domain and abolish the physical interaction of UNC-86 with the LIM homeodomain protein, MEC-3. This results in a failure to maintain mec-3 expression and in loss of expression of the mechanosensor y neuron-specific gene, mec-2, unc-86-dependent expression of genes in othe r neurons is not impaired. We conclude that distinct residues in the POU do main of UNC-86 are involved in modulating UNC-86 activity during its specif ication of different neurons. A structural model of the UNC-86 POU domain, including base pairs and amino acid residues required for MEC-3 interaction , revealed that P145 and L195 are part of a hydrophobic pocket which is sim ilar to the OCA-B-binding domain of the mammalian POU protein, Oct-1.