Frizzleds (FZDs) are unconventional G protein-coupled receptors that belong to the class Frizzled. They are bound and activated by the Wingless/Int-1 lipoglycoprotein (WNT) family of secreted lipoglycoproteins. To date, mechanisms of signal initiation and FZD-G protein coupling remain poorly understood. Previously, we showed that FZD(6) assembles withG alpha(i1)/G alpha(q) (but not withG alpha(s), G alpha(o) and G alpha(12/13)), and that these inactive-state complexes are dissociated by WNTs and regulated by the phosphoprotein Dishevelled (DVL). Here, we investigated the inactive-state assembly of heterotrimeric G proteins with FZD(4), a receptor important in retinal vascular development and frequently mutated in Norrie disease or familial exudative vitreoretinopathy. Live-cell imaging experiments using fluorescence recovery after photobleaching show that human FZD(4) assembles-in a DVL-independent manner-with G alpha(12/13) but not representatives of other heterotrimeric G protein subfamilies, such as G alpha(i1), G alpha(o), G alpha(s), andG alpha(q). The FZD(4)-Gprotein complex dissociates upon stimulation with WNT-3A, WNT-5A, WNT-7A, and WNT-10B. In addition, WNT-induced dynamic mass redistribution changes in untransfected and, even more so, in FZD(4) green fluorescent protein-transfected cells depend on G alpha(12/13). Furthermore, expression of FZD(4) and G alpha(12) or G alpha(13) in human embryonic kidney 293 cells induces WNT-dependent membrane recruitment of p115-RHOGEF (RHO guanine nucleotide exchange factor, molecular weight 115 kDa), a direct target of G alpha(12/13) signaling, underlining the functionality of an FZD(4)-G alpha(12/13)-RHO signaling axis. In summary, G alpha(12/13)-mediatedWNT/FZD(4) signaling through p115-RHOGEF offers an intriguing and previously unappreciated mechanistic link of FZD(4) signaling to cytoskeletal rearrangements and RHO signaling with implications for the regulation of angiogenesis during embryonic and tumor development.