klp-20 encodes one of the two motor subunits of heterotrimeric kinesin-II, the anterograde intraflagellar transport (IFT) motor of Caenorhabditis elegans. It is a member of the kinesin-2 subfamily with an N-terminal kinesin motor domain (P-loop ATPase) and a C-terminal coiled-coil stalk. klp-20 does not act alone: it heterodimerizes through its C-terminal stalk with the second motor subunit klp-11, and this heterodimer associates with the non-motor accessory subunit kap-1 to form the heterotrimeric kinesin-II holoenzyme (klp-11/klp-20/kap-1). Heterodimerization with klp-11 is required to generate a processive motor and to bind kap-1, which links the motor to IFT cargo. As part of kinesin-II the protein is an ATP-driven, microtubule plus-end-directed motor that, together with the homodimeric kinesin-2 motor osm-3, powers anterograde IFT along the middle (doublet) segment of sensory-neuron cilia, building and maintaining the ciliary axoneme. It localizes to sensory cilia, including the ciliary base and transition zone.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003777 microtubule motor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) support for microtubule motor activity, redundant with the experimental IDA below (PMID:17000880). Correct core molecular function of the kinesin motor domain. The more specific term is plus-end-directed microtubule motor activity (GO:0008574), captured in core_functions. Supporting Evidence: PMID:17000880 indicating that Mg-ATP is the preferred substrate for kinesin-2 motors |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Broad cytoplasmic localization consistent with UniProt (Cytoplasm, cytoskeleton by similarity), but uninformative relative to the specific ciliary localization. Keep as non-core context. |
| GO:0008017 microtubule binding | IBA GO_REF:0000033 | ACCEPT | Summary: Microtubule binding is intrinsic to the kinesin motor domain and required for the motor to engage its track. Correct and consistent with plus-end-directed motor activity; retained as a supporting molecular function. |
| GO:0060271 cilium assembly | IBA GO_REF:0000033 | ACCEPT | Summary: Kinesin-II (with klp-20 as a motor subunit) and osm-3 move IFT particles that redundantly build the sensory cilium foundation, so involvement in cilium assembly is well supported. Accept as a core biological process. Supporting Evidence: PMID:17000880 either motor but not both being dispensable for this function |
| GO:0005871 kinesin complex | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Correct but general parent of the specific complex membership. klp-20 is a subunit of the (axonemal heterotrimeric) kinesin-II complex; the specific terms GO:0016939/GO:0030993 are preferred. Keep as non-core. |
| GO:0005874 microtubule | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: The microtubule is the motor's track rather than a distinct cellular location of the protein. Consistent with function but non-core; the informative location is the cilium. |
| GO:0016887 ATP hydrolysis activity | IBA GO_REF:0000033 | ACCEPT | Summary: ATP hydrolysis powers the kinesin motor; the motor domain of klp-20 contains the P-loop (Walker A) ATPase site, and purified kinesin-II shows Mg-ATP-dependent motility with Michaelis-Menten kinetics (PMID:17000880). Accept as part of the core motor mechanism. Supporting Evidence: PMID:17000880 kinesin-II–driven motility conformed to Michaelis-Menten kinetics |
| GO:0008089 anterograde axonal transport | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: This term is a phylogenetic transfer from kinesin-II orthologs (KIF3) that act in neuronal axonal transport. In C. elegans the documented role of klp-20/kinesin-II is anterograde intraflagellar (ciliary) transport, not classical axonal cargo transport; there is no experimental evidence for a klp-20 axonal-transport role in worm. Treated as an over-propagated electronic inference; the accurate specific term is intraciliary anterograde transport (GO:0035720). Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: CONTEXT OR TISSUE MISMATCH GRANULARITY MISMATCH Sources checked: FB:FBgn0004380 · Klp64D (Drosophila kinesin-II KIF3A ortholog) SUPPORTS SOURCE BUT NOT TARGET Kinesin-II orthologs act in neuronal anterograde axonal transport, but the documented C. elegans klp-20 role is anterograde intraciliary transport, not axonal cargo transport; the axon-specific term does not transfer. PANTHER:PTN000650181 · kinesin-2 anterograde-transport node SUPPORTS SOURCE BUT NOT TARGET Node-level anterograde-transport inference; the ciliary (GO:0035720) rather than axonal (GO:0008089) child is the correct scoping for klp-20. |
| GO:0003777 microtubule motor activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (InterPro/ARBA) support for microtubule motor activity, redundant with the experimental IDA (PMID:17000880) and the IBA above. Correct core molecular function. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: ATP binding via the conserved P-loop/Walker A motif of the kinesin motor domain (UniProt BINDING 91..98). Correct molecular-mechanism support for the ATP-driven motor; retained as supporting. |
| GO:0005856 cytoskeleton | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: General cytoskeletal localization from an electronic subcellular-location rule. Consistent with a microtubule motor but non-specific; the informative location is the cilium. |
| GO:0005929 cilium | IEA GO_REF:0000120 | ACCEPT | Summary: Ciliary localization is well established: klp-20/kinesin-II functions in sensory cilia and UniProt records SUBCELLULAR LOCATION cilium (localizing to the base and transition zone; PMID:28479320). Accept as a core cellular location. |
| GO:0007018 microtubule-based movement | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic support for microtubule-based movement, redundant with the experimental IDA (PMID:17000880). Correct but general; the specific process is intraciliary anterograde transport (GO:0035720). Accept. |
| GO:0008017 microtubule binding | IEA GO_REF:0000002 | ACCEPT | Summary: Electronic (InterPro) support for microtubule binding, redundant with the IBA above and intrinsic to the motor domain. Accept as supporting molecular function. |
| GO:0032991 protein-containing complex | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Root-level complex membership from an ARBA rule. True (klp-20 is part of kinesin-II) but uninformative; the specific complex terms GO:0016939 and GO:0030993 supersede it. Over-annotated at this level of generality. |
| GO:1904115 axon cytoplasm | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Inferred logically (GO_REF:0000108) from the anterograde axonal transport annotation (GO:0008089), which is itself an over-propagated phylogenetic transfer. klp-20 acts in sensory cilia in C. elegans, not documented axoplasm; this location is a downstream consequence of the over-annotated axonal-transport term. |
| GO:0007018 microtubule-based movement | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: ComplexPortal NAS annotation for microtubule-based movement, supported by the biophysical characterization of the KLP-11/KLP-20 heterodimer motor. Correct but general relative to intraciliary anterograde transport; accept. Supporting Evidence: PMID:20498083 One motor domain is unprocessive as a homodimer |
| GO:0016939 kinesin II complex | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: klp-20 is a subunit of the kinesin II complex (the klp-11/klp-20/kap-1 heterotrimer). Directly supported: heterodimerization of klp-20 with klp-11 is required to bind kap-1 (PMID:20498083), and the purified heterotrimer was characterized in PMID:17000880. Accept as core complex membership; the axonemal term GO:0030993 is the most specific. Supporting Evidence: PMID:20498083 heterodimerization is necessary to bind KAP1, the in vivo link between motor and |
| GO:0035720 intraciliary anterograde transport | NAS PMID:20498083 Regulation of a heterodimeric kinesin-2 through an unprocess... | ACCEPT | Summary: Anterograde IFT is the core biological process of kinesin-II. As a kinesin-II motor subunit, klp-20 drives base-to-tip transport of IFT particles along sensory cilia together with osm-3. Accept as a core process. Supporting Evidence: PMID:17000880 two anterograde IFT motors called kinesin-II and OSM-3 |
| GO:0003777 microtubule motor activity | IDA PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: Direct experimental (IDA) annotation from characterization of purified kinesin-II, which contains klp-20 as one of its two motor subunits and moves microtubules in an ATP-dependent manner. This is the strongest evidence for the core molecular function. The most specific term is plus-end-directed microtubule motor activity (GO:0008574), used in core_functions. Supporting Evidence: PMID:17000880 the KLP-11, KAP-1, and KLP-20 subunits elute as a monodisperse heterotrimeric complex |
| GO:0007018 microtubule-based movement | IDA PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: Direct experimental annotation for microtubule-based movement from the kinesin-II gliding assays. Correct; general relative to intraciliary anterograde transport but experimentally solid. Accept. Supporting Evidence: PMID:17000880 indicating that Mg-ATP is the preferred substrate for kinesin-2 motors |
| GO:0030993 axonemal heterotrimeric kinesin-II complex | IPI PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | ACCEPT | Summary: Direct physical-interaction (IPI) evidence that klp-20 is a subunit of the axonemal heterotrimeric kinesin-II complex, purified as a monodisperse heterotrimer of KLP-11/KLP-20/KAP-1 (with WormBase klp-11 and kap-1 as the with/from partners). This is the most specific and best-supported complex-membership term and the core cellular-component annotation. Supporting Evidence: PMID:17000880 consisting of 1 mol each of its subunits KLP-11, KLP-20, and KAP-1 with a native molecular mass of 287 kD |
| GO:0008574 plus-end-directed microtubule motor activity | IDA PMID:17000880 Mechanism of transport of IFT particles in C. elegans cilia ... | NEW | Summary: Proposed refinement of the experimental microtubule motor activity annotation (GO:0003777, IDA, PMID:17000880) to the more specific plus-end-directed microtubule motor activity. Kinesins are plus-end-directed motors and kinesin-II (with klp-20) drives anterograde (base-to-tip, plus-end-directed) IFT; the purified motor moves microtubules in gliding assays. This is the primary core molecular function. Supporting Evidence: PMID:17000880 kinesin-II alone moved MTs at a maximal rate of 0.3 |
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Download this section (compressed HTML)Q: Within the klp-11/klp-20 heterodimer, which motor domain is the processive one and which is unprocessive, and how do the two heads cooperate to set the ~0.5 um/s in vivo velocity of kinesin-II?
Q: How does cargo binding to kap-1 relieve the tail-mediated autoinhibition of the klp-11/klp-20 heterodimer, and what is the specific contribution of the klp-20 tail (residues 444-445 and the 525-550 klp-11 interaction region)?
Experiment: Single-molecule motility (TIRF) comparison of purified klp-20 homodimer, klp-11 homodimer, and klp-11/klp-20 heterodimer with subunit-specific fluorophores, to assign processivity and duty ratio to each subunit and test the asymmetric-autoregulation model.
Hypothesis: klp-20 and klp-11 have different intrinsic processivities, and one specific subunit is the unprocessive, autoregulatory head.
Experiment: In vivo IFT imaging (kymography of fluorescently tagged IFT components) in klp-20 loss-of-function versus osm-3 loss-of-function animals to define the cargoes and axonemal segments whose transport specifically depends on the klp-20-containing kinesin-II motor.
Hypothesis: The klp-20-containing kinesin-II motor is specifically required for anterograde transport of a defined subset of IFT cargoes along the middle (doublet) segment.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Within the klp-11/klp-20 heterodimer it is not resolved which subunit is the "unprocessive" motor domain and which is the processive one — i.e. whether klp-20 itself is the processive or the autoinhibited/unprocessive head, and the residue-level basis of its individual duty ratio.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: It is established that heterodimerization of the two distinct C. elegans kinesin-2 motor domains converts an otherwise unprocessive homodimer into a processive heterodimer, that the unprocessive subunit mediates an asymmetric autoregulation of motor activity, and that the heterodimer must form to bind kap-1 (the cargo link). UniProt annotates two klp-20 residues (444, 445) as possibly required for autoinhibition within the heterodimer.
Significance: Kinesin-2 heterodimerization is the paradigm for how obligate motor heterodimers tune processivity and autoregulation; assigning the processive vs unprocessive role to klp-20 vs klp-11 defines the mechanistic division of labor of the anterograde IFT motor.
What would resolve it: Single-molecule processivity assays on defined homodimeric vs heterodimeric constructs of the klp-20 and klp-11 motor domains, with subunit-specific labeling and head-tracking to assign duty ratio to each subunit.
Provenance (the field's own admissions):
Gap: The direct, klp-20-motor-selected ciliary cargo repertoire is not enumerated: what cargoes the klp-20-containing kinesin-II specifically delivers is inferred through the IFT-A/IFT-B particle rather than measured for the motor itself.
OPEN BIOLOGYCURATION BP_DARK
What is known: Kinesin-II (with klp-20 as a motor subunit) and osm-3 are the two anterograde IFT motors that move IFT particles to build the sensory cilium foundation; kap-1 provides the in vivo link between the motor and cargo.
Significance: Defining the motor-specific cargo would clarify how the two anterograde IFT motors (kinesin-II vs osm-3) divide cargo responsibilities along the ciliary axoneme, which is central to understanding ciliogenesis and ciliopathy.
What would resolve it: Proximity labeling or cargo-trapping proteomics on the klp-20/kap-1 motor in cilia, combined with klp-20 loss-of-function IFT imaging to identify cargoes whose ciliary delivery specifically requires the klp-20-containing motor.
Provenance (the field's own admissions):
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