Third, MAP kinase can efficiently phosphorylate LIN-31 in vitro. Fourth, we have tested the effects of LIN-31 phosphorylation on vulval induction in two ways. We engineered a lin-31 construct that lacked all of the MAP kinase phosphorylation sites, and found that expression of this construct resulted in a dominant Egl phenotype. Next, we inserted the VP16 transactivation domain into the lin-31 gene. Conversely, unphosphorylated LIN-31 may repress the expression of these genes.
Many transcription factors interact with co-factors that can modulate their activity.
In lin-1 mutants, all Pn.p cells adopt induced vulval cell fates. We have also found that LIN-1 is a MAP kinase substrate in vitro. One intriguing possibility is that phosphorylation of these transcription facts might alter their transcriptional activity by regulating heterodimer formation. Experiments to test this hypothesis are underway.