The gene regulatory function of specific homeobox sequences may be augmented by adjacent domains that interact with cofactor proteins. The combinatorial action of the homeoprotein and these accessory proteins then leads to either activation or repression of target genes.
unc-4 encodes a homeoprotein that specifies synaptic input to VA motor neurons. In
unc-4 mutants, VAs are miswired with inputs from interneurons normally reserved for their lineal sister cells, the VB motorneurons. UNC-4 function depends on the simultaneous presence of UNC-37, a ubiquitously expressed Groucho-like protein. In flies, Groucho interacts with specific DNA-binding proteins to repress gene transcription. Thus, we have proposed that UNC-4 and UNC-37 function together to repress target genes that would otherwise lead to the expression of VB-specific traits in VA motor neurons (1). Here we show that UNC-4 contains a C-terminal Engrailed-like repressor domain that physically interacts with UNC-37 to negatively regulate VB-specific genes. The UNC-4 C-terminal region includes an amino acid sequence that is similar to a conserved repressor domain first defined for the Engrailed family of homeoproteins (2). Five different randomly generated
unc-4 mutants contain ELD missense mutations. Three of the five UNC-4 ELD mutations, as well as a missense mutation in the UNC-4 homeodomain, are suppressed by a specific amino acid substitution in UNC-37 (3). The UNC-4 ELD interacts with UNC-37 in the yeast two hybrid assay and specific ELD missense mutations disrupt this interaction (J. Meir et al., this meeting). UNC-4 and UNC-37 are required for repression of at least two VB motor neuron specific genes,
acr-5 and
del-1, in the VA motor neurons (J. Ross et al., this meeting) (4). ACR-5, an alpha subunit of a nicotinic acetylcholine receptor, and DEL-1, a degenerin-like sodium channel, are putative cell surface proteins that may function to distinguish VB from VA motor neurons. Missense mutations in the UNC-4 ELD result in ectopic expression of
acr-5 and
del-1 in VA neurons suggesting that the ELD is indeed functioning as a repressor domain in vivo. Our results are consistent with a recent report that the canonical Engrailed repressor domain (eh-1) physically interacts with fly Groucho and that the repressor function of eh-1 in vivo depends on Groucho activity (5). 1. Pflugrad, A., et al. (1997) Development 124, 1699-1709. 2. Smith and Jaynes (1996) Development 122: 3141-3150. 3. Winnier, A. et al., 1997 International C. elegans Meeting. 4. Ross, J. et al., 1997 International C. elegans Meeting. 5. Jimenez et al. (1997) Genes & Development 11:3072-3082.