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

Background: 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. Results: 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. Conclusions: 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. Developmental Dynamics 243:1074–1085, 2014. © 2014 Wiley Periodicals, Inc.