HOOK2

HOOK2 is a centrosome-associated, microtubule-binding coiled-coil protein that functions as a scaffold linking microtubule organization, cell polarity, and endosomal trafficking. Its conserved N-terminal domain binds microtubules while its divergent C-terminal domain anchors it to the centrosome through direct interaction with centriolin/CEP110 and recruits the PAR6α/aPKC polarity complex, thereby orienting the centrosome and Golgi apparatus during directed cell migration (PMID:17140400, PMID:27624926). HOOK2 also interacts with CENP-F to promote centrosome-specific microtubule nucleation after depolymerization, and participates in the FTS/Hook/FHIP (FHF) complex that engages the HOPS complex to facilitate early-to-late endosomal trafficking (PMID:19793914, PMID:18799622). Additionally, HOOK2 selectively binds MSUT-2/ZC3H14, a CCCH zinc-finger protein implicated in tau neurotoxicity, representing a conserved interaction from C. elegans to humans (PMID:19273536).

1. 1999 High

   Genetic studies in Drosophila established that the Hook protein family is required for endocytic trafficking, with all three protein domains essential for function, thereby defining the ancestral cellular role of Hook proteins.

   Evidence Drosophila loss-of-function alleles and truncation analysis showing failure of ligand accumulation in multivesicular bodies

   PMID:9927460

   Open questions at the time

   • Mammalian Hook2-specific functions not addressed
   • Molecular target of Hook in endocytic pathway not identified
   • Whether Hook acts as a motor adaptor or scaffold was unknown
2. 2001 High

   Biochemical characterization of mammalian Hook proteins revealed their architecture as coiled-coil proteins that bind microtubules via their N-terminus and organelles via divergent C-termini, establishing the conceptual framework for how Hook proteins link organelles to the microtubule cytoskeleton.

   Evidence In vitro organelle-binding assays, overexpression in cultured cells, immunofluorescence showing Hook3 at cis-Golgi

   PMID:11238449

   Open questions at the time

   • Hook2-specific localization and binding partners not determined
   • Functional distinction among Hook1, Hook2, and Hook3 paralogs unknown
3. 2006 High

   Hook2 was localized to the centrosome throughout the cell cycle and shown to bind centriolin/CEP110 directly, with loss-of-function experiments demonstrating that Hook2 is required for radial microtubule organization and post-nocodazole microtubule regrowth — the first evidence that Hook2 has a centrosome-specific structural and functional role distinct from other Hook paralogs.

   Evidence Immunofluorescence with cell-cycle staging, C-terminal domain pulldown binding assay, dominant-negative overexpression, nocodazole washout regrowth assay

   PMID:17140400

   Open questions at the time

   • Upstream signals recruiting Hook2 to centrosomes not identified
   • Whether Hook2 affects cell division or only interphase microtubule organization was unclear
4. 2008 High

   Discovery of the FTS/Hook/FHIP (FHF) complex showed that Hook proteins (including Hook2) form homo- and heterodimers within a ~500-kDa complex that engages the HOPS tethering complex, directly linking Hook proteins to early-to-late endosomal and lysosomal trafficking.

   Evidence Affinity purification/mass spectrometry from HEK293T cells, yeast two-hybrid, Co-IP, siRNA knockdown of FTS with EGF trafficking and lysosomal clustering assays

   PMID:18799622

   Open questions at the time

   • Specific contribution of Hook2 versus Hook1/Hook3 within the FHF complex not dissected
   • Whether centrosomal and endosomal functions of Hook2 are coordinated or independent was unknown
5. 2009 High

   Two studies expanded Hook2's interactome: CENP-F was shown to interact with Hook2 and to be required for centrosome-specific microtubule nucleation, while MSUT-2/ZC3H14 was identified as a selective Hook2-binding partner conserved from nematodes to humans, opening a potential connection to tau-related neurodegeneration.

   Evidence Yeast two-hybrid and Co-IP for CENP-F interaction with CENP-F KO cells and nocodazole washout; yeast two-hybrid and in vitro binding for MSUT-2 selectivity

   PMID:19273536 PMID:19793914

   Open questions at the time

   • Functional consequence of Hook2–MSUT-2 interaction in mammalian neurons not tested
   • How CENP-F and centriolin/CEP110 interactions with Hook2 are spatially or temporally coordinated is unknown
   • Whether Hook2–MSUT-2 interaction is relevant to tau pathology in vivo remains unaddressed
6. 2016 High

   Hook2 was established as a centrosomal scaffold for the PAR6α/aPKC polarity complex, directly linking microtubule organization to cell polarity: Hook2 depletion abolished PAR6α centrosomal recruitment and Golgi reorientation during directed migration, integrating Hook2 into the cell polarity pathway.

   Evidence Co-IP mapping Hook2 C-terminal domain to PAR6α aPKC-binding domain, siRNA knockdown with wound-healing and Golgi reorientation assays, overexpression-induced aggresome formation

   PMID:27624926

   Open questions at the time

   • Whether Hook2-mediated polarity is relevant in vivo (e.g., in epithelial tissues or neurons) is untested
   • Regulation of Hook2 itself (post-translational modifications, turnover) remains uncharacterized

• It remains unknown how Hook2's centrosomal, polarity, endosomal trafficking, and MSUT-2-related functions are coordinated — whether they represent context-dependent uses of the same scaffold or independent parallel pathways — and whether Hook2 has non-redundant roles in vivo in mammalian development or disease.
• No Hook2 knockout mouse or in vivo mammalian model has been reported
• Structural basis of Hook2 interactions (e.g., cryo-EM or crystal structures of Hook2 complexes) is lacking
• Post-translational regulation of Hook2 is uncharacterized