Hook domain found in the Hook-related protein (HkRP) familyThe HkRP family includes Daple, Girdin and Gipie. Daple, also called Dvl-associating protein with a high frequency of leucine residues, or coiled-coil domain-containing protein 88C (CCDC88C), or Hook-related protein 2 (HkRP2), is a novel non-receptor nucleotide exchange factor (GEF) required for activation of guanine nucleotide-binding proteins (G-proteins) during non-canonical Wnt signaling. Girdin, also called Akt phosphorylation enhancer (APE), or coiled-coil domain-containing protein 88A (CCDC88A), or G alpha-interacting vesicle-associated protein (GIV), or Girders of actin filament, or Hook-related protein 1 (HkRP1), is a bifunctional modulator of guanine nucleotide-binding proteins (G proteins). It acts as a non-receptor guanine nucleotide exchange factor which binds to and activates guanine nucleotide-binding protein G(i) alpha subunits. It also acts as a guanine nucleotide dissociation inhibitor for guanine nucleotide-binding protein G(s) subunit alpha GNAS. In addition, Girdin plays an essential role in cell migration. Gipie, also called GRP78-interacting protein induced by ER stress, or coiled-coil domain-containing protein 88B (CCDC88B), or brain leucine zipper domain-containing protein, or Hook-related protein 3 (HkRP3), is a novel actin cytoskeleton-binding protein and Akt substrate that regulates cell migratory responses in various biological contexts. It acts as a positive regulator of T-cell maturation and inflammatory function. As a microtubule-binding protein, Gipie regulates lytic granule clustering and NK cell killing. All family members contain a conserved globular Hook domain which folds as a variant of the helical calponin homology (CH) domain.