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 neurons, which are required for egg‐laying, and six mechanosensory neurons. To investigate 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 mechanosensory neuron‐specific gene, mec ‐ 2 . unc ‐ 86 ‐dependent expression of genes in other neurons is not impaired. We conclude that distinct residues in the POU domain of UNC‐86 are involved in modulating UNC‐86 activity during its specification 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 similar to the OCA‐B‐binding domain of the mammalian POU protein, Oct‐1.