LSY-22 physically interacts with UNC-37. (A) Yeast two-hybrid assay with UNC-37 and LSY-22 fused to the GAL4-activating domain (AD) or DNA-binding domain (BD) shows interaction of these two proteins. Additionally, the assay indicates that LSY-22 is able to interact with itself, and COG-1 is able to interact with UNC-37. (B) Co-immunoprecipitation of UNC-37 and LSY-22 from transgenic C. elegans expressing 2×FLAG-VENUS epitope-tagged LSY-22 (otEx4125). The lysate column indicates whether lysates were prepared from wild-type animals (WT) or from otEx4125 animals. Control refers to beads with non-specific antibody. Endogenous UNC-37 was detected using an anti-UNC-37 antibody (Winnier et al., 1999). (C) Model for UNC-37–LSY-22 interaction. Given the reported Q-rich domain-mediated oligomerization of UNC-37/Groucho in other species, we suggest that the interaction occurs via their respective Q-rich domains, but interaction via other domains, such as the GP domain, is conceivable as well. This might occur in a tetrameric configuration, as previously reported for Groucho (Chen et al., 1998). Our data indicating a lack of interaction between UNC-37 and itself suggest that LSY-22 may be required for UNC-37 co-repressor complex formation. The interaction of UNC-37 and COG-1 might occur via a WD40/EH1 domain interaction, as suggested by crystallographic studies of such domains (Pickles et al., 2002). We propose that a broadly expressed LSY-22–UNC-37 complex interacts in a tissue-specific manner with specific DNA-binding transcription factors to control cell fate decisions; in ASE, this transcription factor is COG-1, in the VA motoneurons it might be the EH1 domain-containing UNC-4 protein (Winnier et al., 1999), and in other cellular contexts it might be any one of the dozens of EH1 domain-containing proteins (marked X) present in C. elegans (Copley, 2005) (or any other transcription factor interacting with UNC-37 in a non-EH1 domain-dependent manner).