KAP-1 (F08F8.3) is the Caenorhabditis elegans kinesin-associated protein, the non-motor third subunit of heterotrimeric kinesin-II (KLP-11/KLP-20/KAP-1), orthologous to vertebrate KIFAP3 (KAP3) and sea urchin KAP115. It is an armadillo-repeat protein that folds into an elongated superhelix. Its central groove binds the paired tails of the KLP-11 and KLP-20 motor subunits, and a C-terminal helical hook binds their heterodimerization segment, so KAP-1 joins only the KLP-11/KLP-20 heterodimer and not either motor alone. KAP-1 is expressed in ciliated sensory neurons. It concentrates at the ciliary transition zone and moves along cilia and dendrites at the same rates as IFT particles. As the subunit that links kinesin-II to IFT trains, it takes part in anterograde intraflagellar transport: kinesin-II carries trains through the ciliary base and transition zone into the proximal axoneme, where the homodimeric kinesin-2 OSM-3 takes over. Mutations at the KAP-1/motor-tail interfaces break up the motor and impair IFT. Retrograde return of KAP-1 depends on IFT-dynein (CHE-3). In the AWB chemosensory neurons, whose cilia are forked, kap-1 expression depends on the forkhead factor FKH-2, and the OSM-3 kinesin moves independently of kinesin-II.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003777 microtubule motor activity | IDA PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: Purified recombinant kinesin-II (KLP-11/KLP-20/KAP-1 heterotrimer) drives microtubule gliding in vitro; KAP-1 is the non-motor subunit of this motor. Reason: The contributes_to qualifier is right. KAP-1 has no motor domain, but it is a stoichiometric subunit of the purified heterotrimer whose motor activity was measured. The ATPase and stepping are done by KLP-11 and KLP-20. Supporting Evidence: PMID:17000880 the KLP-11, KAP-1, and KLP-20 subunits elute as a monodisperse heterotrimeric complex PMID:17000880 purified kinesin-II and OSM-3 cooperate to generate movement similar to that seen along the cilium |
| GO:0005634 nucleus | IEA GO_REF:0000117 | REMOVE | Summary: ARBA machine-learning prediction of nuclear localization. Reason: No evidence places worm KAP-1 in the nucleus. KAP::GFP is seen in ciliated sensory neurons at the transition zone, along the cilium and in dendrites. The protein works as the cargo-binding subunit of a ciliary kinesin. This electronic inference has no support and goes against what is known about where KAP-1 acts. Supporting Evidence: PMID:10545497 where they appear to concentrate at the base of the transition zones, which correspond to the basal bodies of motile and sensory cilia |
| GO:0005871 kinesin complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO (KAP3 family, IPR008658) mapping to kinesin complex. Reason: Correct but general. KAP-1 is the third subunit of heterotrimeric kinesin-II, and the more specific kinesin II complex terms are also annotated. Supporting Evidence: PMID:17000880 the KLP-11, KAP-1, and KLP-20 subunits elute as a monodisperse heterotrimeric complex |
| GO:0005930 axoneme | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that KAP-1 acts in the ciliary axoneme. Reason: Correct. Kinesin-II carries IFT trains along the proximal axoneme, and KAP::GFP moves along sensory cilia at IFT rates. Supporting Evidence: PMID:10545497 The average velocities of intraflagellar transport of OSM-1::GFP, OSM-6::GFP, and KAP::GFP, were very similar in a given direction |
| GO:0007018 microtubule-based movement | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that KAP-1 acts in microtubule-based movement. Reason: Correct; KAP-1 is a subunit of a microtubule motor complex. The more informative process is intraciliary anterograde transport. Supporting Evidence: PMID:17000880 purified kinesin-II and OSM-3 cooperate to generate movement similar to that seen along the cilium |
| GO:0007018 microtubule-based movement | IDA PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: Purified kinesin-II containing KAP-1 drives microtubule gliding in vitro. Reason: Supported by the gliding assays with the purified heterotrimer. KAP-1 contributes as a subunit, not as a motor. Supporting Evidence: PMID:17000880 purified kinesin-II and OSM-3 cooperate to generate movement similar to that seen along the cilium |
| GO:0007018 microtubule-based movement | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: ComplexPortal (kinesin-II complex) assertion of microtubule-based movement. Reason: Correct at the complex level; duplicates the IDA and IBA rows. |
| GO:0016939 kinesin II complex | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that KAP-1 is part of the kinesin II complex. Reason: Core complex membership. Biochemistry shows KAP-1 co-purifies with KLP-11 and KLP-20 as a stoichiometric heterotrimer, and a structure shows KAP-1 binds the paired motor tails. Supporting Evidence: PMID:17000880 the KLP-11, KAP-1, and KLP-20 subunits elute as a monodisperse heterotrimeric complex PMID:40707480 The two motor tails fold together and are co-recognized by the central groove of KAP-1. |
| GO:0016939 kinesin II complex | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: ComplexPortal assertion that KAP-1 is part of the kinesin II complex. Reason: Core complex membership. The cited paper shows the KLP-11/KLP-20 heterodimer is needed to bind KAP1. Supporting Evidence: PMID:20498083 heterodimerization is necessary to bind KAP1, the in vivo link between motor and cargo |
| GO:0019894 kinesin binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO (KAP3 family) mapping to kinesin binding. Reason: Correct and well supported in the worm. The KAP-1 armadillo-repeat superhelix binds the paired tails of KLP-11 and KLP-20 in its central groove, and its C-terminal helical hook binds their heterodimerization segment. Mutating these interfaces breaks up the motor and impairs IFT. This is KAP-1's defining molecular activity. Supporting Evidence: PMID:40707480 The two motor tails fold together and are co-recognized by the central groove of KAP-1. PMID:40707480 Mutations in the interfaces between KAP-1 and the two tails disrupt the heterotrimeric kinesin-2 complex and impair kinesin-2-mediated intraflagellar transport. |
| GO:0030993 axonemal heterotrimeric kinesin-II complex | IPI PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: KAP-1 co-purifies with KLP-11 and KLP-20 as a monodisperse heterotrimer (IPI with KLP-11 and KLP-20). Reason: Core complex membership, directly shown. The axonemal heterotrimeric kinesin-II complex is the most specific CC term for KAP-1. Supporting Evidence: PMID:17000880 the KLP-11, KAP-1, and KLP-20 subunits elute as a monodisperse heterotrimeric complex |
| GO:0035720 intraciliary anterograde transport | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: ComplexPortal assertion that kinesin-II, including KAP-1, mediates anterograde IFT. Reason: Core process. Kinesin-II carries IFT trains through the ciliary base and transition zone into the proximal axoneme, where OSM-3 takes over. KAP-1 is the subunit that links the motor to the trains, and mutations at the KAP-1/motor-tail interface impair kinesin-2-mediated IFT. Supporting Evidence: PMID:26523365 kinesin-II transports IFT trains through the ciliary base and transition zone to a 'handover zone' on the proximal axoneme PMID:40707480 Mutations in the interfaces between KAP-1 and the two tails disrupt the heterotrimeric kinesin-2 complex and impair kinesin-2-mediated intraflagellar transport. PMID:20498083 heterodimerization is necessary to bind KAP1, the in vivo link between motor and cargo |
| GO:0035869 ciliary transition zone | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that KAP-1 acts at the ciliary transition zone. Reason: Correct; KAP::GFP concentrates at the transition zone, through which kinesin-II imports IFT trains. Supporting Evidence: PMID:10545497 where they appear to concentrate at the base of the transition zones, which correspond to the basal bodies of motile and sensory cilia PMID:26523365 kinesin-II transports IFT trains through the ciliary base and transition zone to a 'handover zone' on the proximal axoneme |
| GO:0035869 ciliary transition zone | IDA PMID:10545497 Role of a class DHC1b dynein in retrograde transport of IFT ... | ACCEPT | Summary: KAP::GFP concentrates at the base of the transition zones of amphid and phasmid cilia. Reason: Directly observed, and the transition zone is where kinesin-II acts in importing trains. Supporting Evidence: PMID:10545497 where they appear to concentrate at the base of the transition zones, which correspond to the basal bodies of motile and sensory cilia |
| GO:0044782 cilium organization | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that KAP-1 acts in cilium organization. Reason: Acceptable. The IBA node reflects the conserved need for heterotrimeric kinesin-2 to build cilia. In C. elegans, OSM-3 largely compensates for loss of kinesin-II in amphid channel cilia, so the requirement there is partial. That is redundancy within the lineage, not loss of the function, so the inherited annotation stands. Supporting Evidence: PMID:26523365 kinesin-II transports IFT trains through the ciliary base and transition zone to a 'handover zone' on the proximal axoneme |
| GO:0097730 non-motile cilium | IDA PMID:10545497 Role of a class DHC1b dynein in retrograde transport of IFT ... | ACCEPT | Summary: KAP::GFP is seen in and moves along sensory (non-motile) cilia of amphid and phasmid neurons. Reason: Directly observed; worm sensory cilia are non-motile. Supporting Evidence: PMID:10545497 The average velocities of intraflagellar transport of OSM-1::GFP, OSM-6::GFP, and KAP::GFP, were very similar in a given direction |
Loading supporting content…
Download this section (compressed HTML)Q: Is the KAP-1 N-terminal region, which is flexible and unresolved in the structure, the part that contacts IFT trains, and is this contact regulated by the DYF-5/MAK kinase at the handover zone?
Experiment: Delete or mutate the KAP-1 N-terminal region at the endogenous locus and image kinesin-II, OSM-3 and IFT-B with single-molecule sensitivity to measure train loading and handover.
Hypothesis: The flexible KAP-1 N-terminus links kinesin-II to IFT trains but is not needed for KLP-11/KLP-20 binding.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Which IFT-train subunit KAP-1 binds to attach kinesin-II to trains is not known in C. elegans. KAP-1 is described as the link between motor and cargo, but the train-side partner and the KAP-1 surface that contacts it have not been identified.
OPEN BIOLOGY
What is known: Established: KAP-1 binds the KLP-11/KLP-20 tails (structure) and is needed for a functional kinesin-II in IFT. Unknown: the direct IFT-A or IFT-B binding partner of KAP-1, and how that contact is released at the handover zone where OSM-3 takes over.
Significance: Would explain how kinesin-II hands trains to OSM-3 and then recycles alone to the ciliary base.
What would resolve it: Crosslinking mass spectrometry or reconstitution of KAP-1 with purified IFT-A and IFT-B subcomplexes, followed by in vivo tests of mutations at the mapped interface.
Provenance (the field's own admissions):
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)