WRM-1 Activates the LIT-1 Protein Kinase to Transduce Anterior/Posterior Polarity Signals in C. elegans

During C. elegans development, Wnt/WG signaling is required for differences in cell fate between sister cells born from anterior/posterior divisions. A β-catenin-related gene, wrm-1, and the lit-1 gene are effectors of this signaling pathway and appear to downregulate the activity of POP-1, a TCF/LEF–related protein, in posterior daughter cells. We show here that lit-1 encodes a serine/threonine protein kinase homolog related to the Drosophila tissue polarity protein Nemo. We demonstrate that the WRM-1 protein binds to LIT-1 in vivo and that WRM-1 can activate the LIT-1 protein kinase when coexpressed in vertebrate tissue culture cells. This activation leads to phosphorylation of POP-1 and to apparent changes in its subcellular localization. Our findings provide evidence for novel regulatory avenues for an evolutionarily conserved Wnt/WG signaling pathway.