HkRP3 Is a Microtubule-Binding Protein Regulating Lytic Granule Clustering and NK Cell Killing

Abstract

NK cells provide host defense by killing viral-infected and cancerous cells through the secretion of preformed lytic granules. Polarization of the lytic granules toward the target cell is dependent on an intact microtubule (MT) network as well as MT motors. We have recently shown that DOCK8, a gene mutated in a primary immunodeficiency syndrome, is involved in NK cell killing in part through its effects on MTOC polarization. In this study, we identified Hook-related protein 3 (HkRP3) as a novel DOCK8- and MT-binding protein. We further show that HkRP3 is present in lytic granule fractions, interacts with the dynein motor complex and MTs. Significantly, depletion of HkPR3 impaired NK cell cytotoxicity, which could be attributed to a defect in not only MTOC polarity, but also impaired clustering of lytic granules around the MTOC. Our results demonstrate an important role for HkRP3 in regulating the clustering of lytic granules and MTOC repositioning during the development of NK cell-mediated killing.

Origins of the cytolytic synapse

“…The molecular basis of these defects arises from the interaction of DOCK8 with WASP and talin 63 , which are important regulators of actin turnover. More recently a novel DOCK8-interacting protein, Hook-related protein 3 (HKRP3; also known as CCDC88B) has been identified that can bind directly to microtubules and can immunoprecipitate dynein from NK cell lysates 64 .…”

Recent Advances in DOCK8 Immunodeficiency Syndrome

“…In T cells, B cells, and NK cells, absence of DOCK8 results in abnormalities in F-actin polymerization at the immunological synapse (39, 40, 42, 51) (Figure 3b). At least during NK cell cytolysis, DOCK8 also regulates microtubule polarization through its interaction via Hook-related protein 3 (HkRNP3) with microtubules and the dynein motor complex (57). During interstitial migration, absence of DOCK8, or of downstream CDC42 or PAK1/2, also leads to abnormalities in F-actin and microtubule structures concurrent with lymphocyte deformation (37).…”

CCDC88B is required for pathogenesis of inflammatory bowel disease

“…The function of these proteins is unclear, but they seem to functionally couple elements of the cytoskeleton with different cellular processes, such as vesicular transport and cell movement 6 . Ccdc88b has been shown to interact with the CDC42 guanine nucleotide exchange factor DOCK8 7 . Human DOCK8 mutations cause primary immunodeficiencies associated with perturbed migration, altered function of myeloid and NK cells 8 , 9 .…”