LIN-12/Notch trafficking and regulation of DSL ligand activity during vulval induction in Caenorhabditis elegans.

A novel mode of crosstalk between the EGFR-Ras-MAPK and LIN-12/Notch pathways occurs during the patterning of a row of vulval precursor cells (VPCs) in Caenorhabditis elegans: activation of the EGFR-Ras-MAPK pathway in the central VPC promotes endocytosis and degradation of LIN-12 protein. LIN-12 downregulation in the central VPC is a prerequisite for the activity of the lateral signal, which activates LIN-12 in neighboring VPCs. Here we characterize cis-acting targeting sequences in the LIN-12 intracellular domain and find that in addition to a di-leucine motif, serine/threonine residues are important for internalization and lysine residues are important for post-internalization trafficking and degradation. We also identify two trans-acting factors that are required for post-internalization trafficking and degradation: ALX-1, a homolog of yeast Bro1p and mammalian Alix and the WWP-1/Su(dx)/Itch ubiquitin ligase. By examining the effects of mutated forms of LIN-12 and reduced wwp-1 or alx-1 activity on subcellular localization and activity of LIN-12, we provide evidence that the lateral signal-inhibiting activity of LIN-12 resides in the extracellular domain and occurs at the apical surface of the VPCs.