Hox transcription factors are key regulators of anterior-posterior fates in animals; Hox-mediated specification of neuron fates on the A-P axis is essential to nervous system organization and function. In the ventral nerve cord of C. elegans males, fates of the nine male-specific CP neurons depend on the activities of the Hox proteins LIN-39 and MAB-5, which are expressed in partially overlapping domains. LIN-39 defines fates of anterior CPs 1-4, including expression of the neurotransmitter genes
tph-1 and
flp-22; MAB-5 defines fates of posterior CPs 7-9, including expression of
flp-21. In the midbody, LIN-39 and MAB-5 together define fates of CPs 5-6, which express
tph-1, but not
flp-22 or
flp-21. In
mab-5(
e1751gf) mutants, MAB-5 is overexpressed in the anterior ventral cord, leading CPs 1-4 to adopt normally CP 5-6-specific traits: CPs 1-4 lack
flp-22::gfp expression and express
tph-1::mCherry at high levels normally seen in CPs 5-6. This is consistent with the idea that, together, LIN-39 and MAB-5 are sufficient to define CP 5-6 fate. How are LIN-39 and MAB-5 activities coordinated to define a unique neuron fate in their region of co-expression? The
lin-39(
ccc16) mutation sheds light on this question.
lin-39(
ccc16) specifically alters CPs 5-6, such that CPs 5-6 express posterior (CP 7-9-like) traits. In contrast, fates of CPs 1-4, which also normally depend on LIN-39 activity, are relatively unaffected by
lin-39(
ccc16). We hypothesize that the protein encoded by
lin-39(
ccc16), which is predicted to lack sequences C-terminal to the homeodomain, is able to specify CP fates in the absence of MAB-5 (i.e., in CPs 1-4), but is unable to interact with MAB-5 to define CP 5-6 fates. Consistent with this,
lin-39(
ccc16) males express
tph-1::mCherry only in CPs 1-4, whereas
lin-39(
ccc16)
mab-5(0) mutants express
tph-1::mCherry in CPs 1-6, suggesting that LIN-39
(ccc16) is functional only in MAB-5's absence. To test whether sequences C-terminal to the LIN-39 homeodomain are necessary for interactions among LIN-39, MAB-5, and TALE cofactor CEH-20we used bimolecular fluorescence complementation to query protein interactions in vivo. Preliminary results suggest that full-length LIN-39 interacts with MAB-5 and CEH-20, but that LIN-39
(ccc16) fails to interact with MAB-5. We hypothesize that sequences C-terminal to LIN-39's homeodomain mediate formation of a complex that contains LIN-39 and MAB-5, allowing them to coregulate targets to define CP 5-6 fates.