Cell-cell communication via Wnt signals represents a fundamental means by which animal development and homeostasis are controlled. The identification of components of the Wnt pathway is reaching saturation for the transduction process in receiving cells but is incomplete concerning the events occurring in Wnt-secreting cells. Here, we describe the discovery of a novel Wnt pathway component, Wntless (Wls/Evi), and show that it is required for Wingless-dependent patterning processes in Drosophila, for MOM-2-governed polarization of blastomeres in C. elegans, and for Wnt3a-mediated communication between cultured human cells. In each of these cases, Wls is acting in the Wnt-sending cells to promote the secretion of Wnt proteins. Since loss of Wls function has no effect on other signaling pathways yet appears to impede all the Wnt signals we analyzed, we propose that Wls represents an ancient partner for Wnts dedicated to promoting their secretion into the extracellular milieu.
