Function of the Ryk intracellular domain in C. elegans vulval development

Ryk is a subfamily of receptor tyrosine kinases, which along with Frizzled and Ror, function as Wnt receptors. Vertebrate Ryk intracellular domain (ICD) is released from the cell membrane by a proteolytic cleavage in the transmembrane region and localizes to the nucleus. In C. elegans, Ryk is encoded by the lin-18 gene and regulates the polarity of the P7.p vulval cell.

Based on Western blots, we were unable to detect the presence of the cleaved LIN-18 ICD fragment. Functional assays found that LIN-18 intracellular domain is not absolutely required for LIN-18 function, consistent with previous results. However, overexpression of the LIN-18 intracellular domain fragment (LIN-18ICD) weakly enhanced the phenotype of lin-18 loss-of-function mutants. Furthermore, this activity was specific to the serine-rich juxtamembrane region. We also found that the nuclear localization of LIN-18ICD fragment can be regulated by Wnt pathway components including CAM-1/Ror, and by PAR-5/14-3-3.

Release of LIN-18ICD by cleavage at the membrane is not the main mechanism of LIN-18 signaling in vulval cells. However, our results suggest that LIN-18 intracellular domain interacts with Wnt pathway components and a 14-3-3 protein and likely plays a minor role in LIN-18 signaling.