KIFAP3

KIFAP3 (KAP3) is the non-motor adaptor subunit of the heterotrimeric kinesin-2 complex, essential for targeting the KIF3A/KIF3B motor to sites of intracellular transport and for coupling it to specific cargoes. KAP3 contains nine Armadillo repeats, binds the coiled-coil tail domain of KIF3 subunits without affecting intrinsic ATPase activity, and is required for efficient anterograde intraflagellar transport and flagellar/cilia assembly (PMID:8710890, PMID:15616187). KAP3 serves as a cargo-recruitment platform: it engages cargo adaptors such as APC and TRIM46, and its binding occludes an autoinhibitory beta-hairpin motif in the KIF3 tails, thereby activating processive motility (PMID:41910726). In neurons, KAP3-dependent kinesin-2 transports choline acetyltransferase along motor axons, and sequestration of KAP3 by misfolded SOD1 impairs this transport in ALS models; loss of KAP3 in epithelial cells disrupts post-Golgi laminin transport and cell–cell adhesion (PMID:19088126, PMID:35322078).

1. 1996 High

   Identifying KAP3 as a non-motor subunit of kinesin-2 established that this motor operates as a heterotrimer rather than a simple heterodimer, and that KAP3 associates with the motor tail without altering its enzymatic activity.

   Evidence Immunoprecipitation, microsequencing, baculovirus/Sf9 reconstitution, and motor ATPase assays from mouse testis

   PMID:8710890

   Open questions at the time

   • Cargo identity unknown at this stage
   • No structural information on the KAP3–tail interface
   • Cellular function of the two splice isoforms (KAP3A/B) unresolved
2. 1996 Medium

   Discovery of nine Armadillo repeats in KAP3 and its interaction with SmgGDS, regulated by Src phosphorylation, suggested KAP3 functions as a regulated protein–protein interaction scaffold linking signaling to motor transport.

   Evidence Yeast two-hybrid, recombinant protein binding assays, in vitro Src kinase assay

   PMID:8900189

   Open questions at the time

   • Physiological relevance of SmgGDS interaction not tested in cells
   • In vivo phosphorylation by Src not demonstrated
   • Whether Src regulation affects kinesin-2 transport in cells unknown
3. 1998 Medium

   Identification of a ternary complex between KAP3, KIF3B, and the condensin subunit HCAP in nuclear fractions raised the possibility that kinesin-2 functions extend to chromosome-associated processes.

   Evidence Yeast two-hybrid, co-immunoprecipitation from nuclear extracts

   PMID:9506951

   Open questions at the time

   • No functional consequence of the HCAP–KAP3 interaction demonstrated
   • Nuclear role of kinesin-2 not validated by loss-of-function
   • Interaction not confirmed in other cell types
4. 2004 High

   A temperature-sensitive KAP mutation in Chlamydomonas demonstrated that KAP3 is required in vivo for targeting kinesin-2 to the basal body and for maintaining anterograde IFT frequency, directly linking the adaptor subunit to cilium assembly.

   Evidence Temperature-sensitive mutant, video-enhanced DIC microscopy of IFT, transgenic rescue

   PMID:15616187

   Open questions at the time

   • Whether KAP3 directly contacts IFT-B particles or uses an intermediate adaptor not determined
   • Mechanism by which the C-terminal domain mediates basal-body targeting unclear
   • Applicability to mammalian ciliogenesis not tested
5. 2008 High

   Demonstrating that misfolded SOD1 sequesters KAP3, thereby impairing kinesin-2-dependent axonal transport of choline acetyltransferase, established KAP3 as a mechanistic link between protein misfolding and motor neuron transport deficits in ALS.

   Evidence Co-IP from spinal cord, ChAT transport assay, KAP3 overexpression rescue in NG108-15 cells, human FALS tissue immunostaining

   PMID:19088126

   Open questions at the time

   • Whether KAP3 sequestration is sufficient to cause motor neuron degeneration not shown
   • Direct binding interface between SOD1 and KAP3 not mapped
   • Other kinesin-2 cargoes affected by SOD1-mediated KAP3 loss not catalogued
6. 2022 High

   CRISPR knockout of KAP3 in gastric carcinoma cells revealed its requirement for post-Golgi laminin transport and RhoA-dependent circumferential actomyosin organization, extending KAP3 function to epithelial basement membrane integrity and cell–cell adhesion.

   Evidence CRISPR/Cas9 KO, immunofluorescence, RhoA activity assay, basement membrane formation assay

   PMID:35322078

   Open questions at the time

   • Whether KAP3 directly binds laminin-containing vesicles or acts through an intermediate unknown
   • Whether RhoA inactivation is a direct or secondary consequence of transport failure not resolved
   • Relevance to normal epithelial morphogenesis not demonstrated
7. 2024 High

   Structural and single-molecule work revealed that KIF3 tails contain a conserved beta-hairpin motif that autoinhibits the motor, and KAP3 binding creates a platform where cargo adaptors occlude this motif to activate processive motility, defining KAP3 as an allosteric activator of kinesin-2.

   Evidence Cryo-EM, single-molecule motility assays, mutagenesis (preprint)

   Open questions at the time

   • Peer review pending
   • Full set of physiological cargo adaptors that exploit this activation mechanism not catalogued
   • Whether autoinhibition relief differs between KIF3A- and KIF3B-enriched assemblies not determined
8. 2026 Medium

   Discovery of a KIF3B-enriched, KAP3-associated kinesin-2 assembly that preferentially transports TRIM46 to the axon initial segment showed that kinesin-2 exists in distinct compositional states with cargo selectivity.

   Evidence Co-immunoprecipitation, cellular fractionation, live-cell imaging, structural analysis

   PMID:41910726

   Open questions at the time

   • Stoichiometry of the KIF3B-enriched complex not fully resolved
   • How the tail conformation difference is regulated remains unclear
   • Whether other neuronal cargoes use the same KIF3B-enriched assembly unknown

• The full structural basis for how KAP3 selectively recognizes different cargoes through its Armadillo repeat domain, and how post-translational modifications such as Src phosphorylation regulate this selectivity in vivo, remains to be established.
• No high-resolution structure of KAP3 Armadillo domain bound to a cargo adaptor peptide
• In vivo phospho-regulation of KAP3 not demonstrated
• Relative contributions of KAP3A vs KAP3B splice isoforms to cargo specificity unknown