Required for activation of guanine nucleotide-binding proteins (G proteins) during non-canonical Wnt signaling (PubMed:26126266). Binds to ligand-activated Wnt receptor FZD7, displacing DVL1 from the FZD7 receptor and leading to inhibition of canonical Wnt signaling (PubMed:26126266). Acts as a non-receptor guanine nucleotide exchange factor by also binding to guanine nucleotide-binding protein G(i) alpha (Gi-alpha) subunits, leading to their activation (PubMed:26126266). Binding to Gi-alpha subunits displaces the beta and gamma subunits from the heterotrimeric G protein complex, triggering non-canonical Wnt responses such as activation of RAC1 and PI3K-AKT signaling (PubMed:26126266). Promotes apical constriction of cells via ARHGEF18 (PubMed:30948426). . This is the function of ENSG00000015133 (CCDC88C, coiled-coil and HOOK domain protein 88C).