Wntless-Sec12 complex on ER membrane regulates early Wnt secretory vesicle assembly and mature ligand export

Abstract

Wntless (Wls) transports Wnt molecules for secretion, however the cellular mechanism underlying the initial assembly of Wnt secretory vesicles is still not fully defined. Using proteomic and mutagenic analyses of the mammalian Wls, we report a mechanism for formation of early Wnt secretory vesicle on ER membrane. Wls forms a complex with SEC12, an ER membrane-localized GEF activator of SAR1 small GTPase. Compared to palmitoylation-deficient Wnt molecules, binding of mature Wnt to Wls increases Wls-SEC12 interaction and promotes Wls association with SAR1, the key activator of the COPII machinery. Incorporation of Wls into this ER exporting compartment is sensitively affected by Wnt ligand-binding, SEC12-binding of Wls, as well as the structural integrity and potentially the folding of the cytosolic tail of Wls. In contrast, Wls-SEC12 binding is stable, with the interacting interface biochemically mapped to cytosolic segments of individual proteins. Mutant Wls that fails to communicate with COPII machinery cannot effectively support Wnt secretion. These data suggest that formation of early Wnt secretory vesicles is carefully regulated to ensure proper export of functional ligands.