CDK16 (PCTAIRE1/PCTK1) is an X-linked atypical cyclin-dependent serine/threonine protein kinase of the PCTAIRE group (CDK16-18) in the CMGC kinase family. This 421-residue blue whale product is an N-terminally shorter alternative-start isoform that corresponds to residues 76-496 of human CDK16 isoform 1. It keeps the complete kinase domain, the catalytic lysine and HRD aspartate, and every cyclin Y-binding element mapped in the human protein, but it lacks the N-terminal segment that carries the BRSK2 phosphorylation site Ser12. In mammals CDK16 is activated by the membrane-anchored cyclin Y (CCNY) and its relative CCNYL1. Cyclin binding requires a conserved part of the N-terminal extension as well as the kinase domain, and is blocked by phosphorylation of Ser153 (Ser78 in this isoform); 14-3-3 binding to phosphorylated cyclin Y also contributes to activation. Cyclin Y recruits the kinase from the cytoplasm to the cytoplasmic face of the plasma membrane. CDK16 prefers substrates with proline at +1 and a basic residue at +4. Its known substrates include NSF, whose phosphorylation limits NSF oligomerization and modulates regulated secretion, and the autophagy protein WIPI2B in neurons. CDK16 is most abundant in brain, testis and pancreatic islets. It contributes to dendrite development and neurotransmitter or hormone release, and male mice lacking it are infertile with defective sperm.
Summary: True for a protein kinase, but entailed by the ATP binding annotation (GO:0005524) accepted below, which is supported by the PROSITE ATP-binding signature and the conserved catalytic lysine (K119, human K194).
Reason: Nucleotide binding is a correct ancestor of ATP binding, which this review accepts; the general term adds nothing.
Summary: CDK16 is a protein kinase (InterPro Ser/Thr kinase active-site signature). The cyclin-dependent protein serine/threonine kinase activity accepted below is a descendant of this term.
Reason: Correct but entailed by GO:0004693, which this review accepts as the core activity.
Supporting Evidence:
file:BALMU/A0A8B8WEG2/A0A8B8WEG2-uniprot.txt
DR InterPro; IPR008271; Ser/Thr_kinase_AS.
GO:0004674 protein serine/threonine kinase activity
IEA GO_REF:0000120
TOO GENERAL
Summary: The mammalian orthologs phosphorylate serine residues on proteins (NSF Ser569, WIPI2B Ser395, CCNY Ser336). GO:0004693, accepted below, is a descendant of this term.
Reason: Correct but entailed by GO:0004693 cyclin-dependent protein serine/threonine kinase activity, which better describes this cyclin Y-activated kinase.
binds directly to and is phosphorylated by Pctaire1 on serine 569
GO:0004693 cyclin-dependent protein serine/threonine kinase activity
IEA GO_REF:0000120
ACCEPT
Summary: A0A8B8WEG2 is in PANTHER subfamily PTHR24056:SF174 (CDK16) with human Q00536, mouse Q04735 and rat Q63686, and matches the CDD PCTAIRE1 kinase-domain model. Human and mouse CDK16 are activated by cyclin Y: CCNY binding increases activity more than 100-fold, and the kinase-dead K194R mutant is not activated. The whale isoform lacks only the first 75 residues of human isoform 1. Human mapping showed that the first 105 residues are dispensable for CCNY binding, and a recombinant CDK16 lacking its first 106 residues was active only with CCNY. All the mapped cyclin-binding elements (112-121, Ser119, Ser153, Gly158 and the C-terminal 461-473 segment) and the catalytic residues are conserved in this isoform (file:BALMU/A0A8B8WEG2/A0A8B8WEG2-bioinformatics/RESULTS.md).
Reason: The core activity of CDK16, carried over from orthologs in the same PANTHER subfamily with direct biochemical evidence. The cyclin-binding and catalytic determinants are present in this isoform.
Supporting Evidence:
file:BALMU/A0A8B8WEG2/A0A8B8WEG2-uniprot.txt
DR PANTHER; PTHR24056:SF174; CYCLIN-DEPENDENT KINASE 16; 1.
Summary: The protein kinase ATP-binding signature (PROSITE PS00107) is present, and the conserved ATP-binding lysine is at position 119 (human K194; K194R is kinase-dead).
Reason: ATP binding is required for the kinase activity and is supported by the signature and the conserved catalytic lysine.
Summary: This TreeGrafter row comes from PTN000623744. In human CDK16 the matching nucleus IBA is seeded only by Arabidopsis CDKs, i.e. it reflects nuclear cell-cycle CDKs placed at a deep node. The direct evidence for mammalian CDK16 points to the cytoplasm: CDK16-GFP alone was found exclusively in the cytoplasm of 293A cells, and endogenous PCTAIRE1 is cytoplasmic in MIN6 beta cells. CCNY recruits it to the plasma membrane. The nuclear localization in the earlier version of this review came from an unreferenced deep-research statement.
Reason: Inherited from a deep CDK node and not supported by localization studies of CDK16 orthologs, which show cytoplasmic and membrane-recruited protein.
Summary: Mammalian CDK16 is cytoplasmic in transfected 293A cells and in MIN6 beta cells, and a fraction is recruited by myristoylated CCNY to the cytoplasmic face of the plasma membrane.
Reason: Consistent with direct localization data for the orthologs and with the PAINT placement.
Summary: PAINT seeded exocytosis at the CDK16 node from the mouse Cdk16 IMP to PMID:16461345. That study shows that PCTAIRE1 phosphorylates NSF Ser569, which restrains NSF oligomerization. Kinase-dead PCTAIRE1 or NSF-S569A increased stimulated growth hormone release from PC12 cells. The kinase therefore modulates exocytosis through NSF; it is not part of the fusion machinery.
Reason: The evidence describes a regulatory kinase acting on NSF, which fits regulation of exocytosis (GO:0017157) better than the process itself. The phylogenetic basis (same-subfamily mouse ortholog) is sound.
Summary: In the human and PAINT rows this term is colocalizes_with synaptic vesicle, based on rodent PCTAIRE1 associating with NSF and synaptic vesicle proteins in brain. The TreeGrafter row for this whale entry carries located_in instead. Rat PCTAIRE1 also appears as puncta that co-localize with a synaptic marker in mature dendrites. Association with synaptic vesicle proteins is plausible, but membership of the vesicle has not been shown.
Reason: A peripheral, neuron-specific association with synaptic vesicles, inherited from the same-subfamily mouse ortholog. The qualifier should be colocalizes_with, as at the PAINT node, not located_in.
Summary: PAINT seeded this term at the CDK16 node from mouse Cdk16. In hippocampal neurons, PCTAIRE1 is concentrated at growth cones; knockdown abolishes dendrite development, and a mutant that cannot be phosphorylated on Ser95 by CDK5 reduces dendrite complexity. Ser95 is conserved in this isoform (Ser20). The evidence is mainly perturbation, and the substrate through which CDK16 acts in this process is not known.
Reason: Supported by same-subfamily ortholog evidence, but the evidence shows a signalling kinase that modulates dendrite growth by an undefined mechanism, not a component that executes projection development. The participation term is therefore corrected to regulation of neuron projection development (GO:0010975), as in the human CDK16 comparator review; not the core function.
Summary: This row is inferred mechanically from GO:0004693 through an inter-ontology link, not from evidence on CDK16. The documented CDK16 functions are in post-mitotic and secretory cells (neurons, beta cells, sperm). Human CDK16 dissociates from CCNY in mitosis because of Ser153 phosphorylation. Human CDK16 does not currently receive a cell-cycle process IBA (the GO:1901987 IBA still listed in its UniProt record is absent from QuickGO). The cancer-cell substrate claims in the deep-research report have no references and were not verified.
Reason: An ontology-link inference that assumes a canonical cell-cycle CDK role. The literature on this atypical PCTAIRE kinase does not establish that role.
with respect to mitosis, we find that CDK16, in contrast to CDK14/PFTK, does not interact with CCNY due to phosphorylation of Ser153
GO:0000307 cyclin-dependent protein kinase holoenzyme complex
IBA
NEW
Summary: Human CDK16 carries part_of GO:0000307 by IBA and IPI (ComplexPortal, CCNY-CDK16), and mouse/human data show a stable CDK16-CCNY complex that is needed for activity. This whale isoform keeps every cyclin Y-binding element mapped in human CDK16.
Reason: The CDK16-cyclin Y holoenzyme is the active form of the kinase. Its binding determinants are conserved in this isoform, and the subfamily-level PAINT assignment covers it.
It keeps the complete kinase domain, the catalytic residues and all the cyclin Y binding determinants mapped by
Core Functions
Cyclin Y-activated serine/threonine protein kinase. Together with CCNY (or CCNYL1) it forms a holoenzyme that phosphorylates substrates such as NSF and WIPI2B. It acts in the cytoplasm and at the cytoplasmic face of the plasma membrane, where cyclin Y recruits it.
Q: Is the whale alternative-start isoform (A0A8B8WEG2, starting at the equivalent of human Met76) expressed as a protein, and does it differ from the full-length product in regulation, given that it lacks the Ser12 BRSK2 phosphorylation site?
Q: Should the TreeGrafter projection of the CDK16-node synaptic vesicle IBA keep the colocalizes_with qualifier of the source annotation?
Suggested Experiments
Experiment: Express the 421-residue whale isoform and full-length CDK16 with CCNY, and compare their kinase activity on PCTAIRE-tide or NSF, their CCNY binding and their recruitment to the plasma membrane.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
ProtNLM2 made no predictions (GO or subcellular location) for this blue whale protein and returned only the generic name "Protein kinase domain-containing protein". That name is correct but much less specific than the gene review, which identifies the product as an N-terminally shorter alternative-start isoform of CDK16 (PCTAIRE1), a PCTAIRE-group CMGC kinase that retains the complete kinase domain and catalytic residues and acts as a cyclin Y-activated cyclin-dependent protein serine/threonine kinase in the cytoplasm and at the cytoplasmic face of the plasma membrane. With no GO or location output there is nothing to score; the absence of predictions is recorded as assessed.