ALKAL2 (ALK and LTK ligand 2, also called FAM150B or augmentor-alpha) is a small secreted cytokine that acts as an activating ligand for the receptor tyrosine kinases ALK and LTK. Its C-terminal augmentor domain, a three-helix bundle stabilized by two intramolecular disulfides, binds the receptor extracellular region and drives receptor dimerization close to the plasma membrane, where ligand-membrane contacts stabilize the signaling complex. Receptor activation triggers downstream MAP kinase (ERK1/2 and ERK5) signaling and supports neural development and regulation of energy expenditure. The donkey protein A0A9L0J5U7 is the full-length ortholog (86% identity to human, with an invariant augmentor domain); it lacks the primate-specific N-terminal cysteine that forms the interchain disulfide in human ALKAL2.
Summary: Extracellular region from the UniProt subcellular location mapping (Secreted). ALKAL2 has an N-terminal signal peptide and is a secreted cytokine; human ALKAL2 is secreted and purified from conditioned medium.
Reason: The donkey protein carries a predicted 24-residue signal peptide like the human ortholog, and the ligand acts on the extracellular region of ALK/LTK. Extracellular region is the appropriate location for this secreted ligand.
Summary: Plasma membrane from the UniProt subcellular location mapping (Cell membrane). Human ALKAL2 associates with the cell membrane after binding ALK, and ligand-membrane contact stabilizes the active receptor complex.
Reason: ALKAL2 is a soluble secreted ligand without a transmembrane segment or lipid anchor; its plasma membrane association is peripheral and occurs at the cell surface in complex with the receptor. The annotation is supported for the human ortholog but is secondary to its extracellular location.
This orientation is further stabilized by an interaction between the ligand and the membrane.
GO:0030298 receptor signaling protein tyrosine kinase activator activity
IEA GO_REF:0000120
MODIFY
Summary: Activation of the receptor tyrosine kinases ALK and LTK is the defining activity of ALKAL2. Human ALKAL2 (FAM150B) stimulates ALK autophosphorylation and downstream signaling, and structures show it drives ALK/LTK dimerization.
Reason: The essence is right and is the core function: the donkey protein sits in PANTHER subfamily PTHR28676:SF2 (ALK AND LTK LIGAND 2) with experimentally characterised human ALKAL2 (Q6UX46), and its augmentor domain conserves the disulfides and ALK-contact residues. The term is the wrong sibling, however. GO:0030298 is defined for "a receptor signaling protein tyrosine kinase" and sits only under protein tyrosine kinase activator activity, with no signaling receptor activator parent. ALK and LTK are transmembrane receptor tyrosine kinases that ALKAL2 binds on their extracellular region, which is exactly the definition of GO:0030297 transmembrane receptor protein tyrosine kinase activator activity; that term is_a signaling receptor activator activity, and it is the term used for other RTK ligands such as EGF, NGF, VEGFA and EFNA5 (QuickGO, human, 2026-10-21). The ALK-axis usage of GO:0030298 (InterPro2GO mapping for IPR029364, and the human ALKAL1/ALKAL2 IDA rows) appears to be a historical term choice rather than a distinct biological claim; raised as a suggested question.
The donkey sequence A0A9L0J5U7 (160 residues) aligns to human Q6UX46 (152 residues) with 86.18% identity across 152 paired residues, covering 100% of the human sequence.
Summary: ALKAL2 binds directly to the extracellular region of ALK and LTK; this ligand-receptor binding is resolved in crystal and cryo-EM structures of the human complexes.
Reason: Receptor binding is intrinsic to the ligand mechanism and specifies the receptor class; it is an informative binding term rather than generic protein binding, and the receptor-contact residues are conserved in donkey.
The cognate cytokines ALKAL1 and ALKAL2 are monomeric three-helix bundles, yet their binding to ALK and LTK elicits similar dimeric assemblies with two-fold symmetry
Residues mutated in the human ALK-binding studies are conserved
GO:0070374 positive regulation of ERK1 and ERK2 cascade
IEA GO_REF:0000118
KEEP AS NON CORE
Summary: TreeGrafter transfer from the ALKAL2 PANTHER node. Human ALKAL2-activated ALK stimulates ERK1/2 phosphorylation in PC12 and neuroblastoma cells.
Reason: ERK1/2 activation is a downstream consequence of ALK/LTK activation by the ligand rather than a direct activity of ALKAL2, but the ligand is the upstream initiator of this signaling and the effect is experimentally documented for the human ortholog. Kept as a non-core regulatory process.
Summary: TreeGrafter transfer from the ALKAL2 PANTHER node. FAM150B-conditioned medium stimulated ERK5 phosphorylation downstream of ALK in IMR-32 neuroblastoma cells.
Reason: As for ERK1/2, this is a downstream readout of receptor activation by the ligand; supported for human ALKAL2 and conserved by orthology, but non-core.
This response was observed at the level of ALK receptor phosphorylation (pALK-Y1604), and with downstream pERK5
GO:0007169 cell surface receptor protein tyrosine kinase signaling pathway
ISS file:EQUAS/ALKAL2/ALKAL2-bioinformatics/ALKAL2-human-Q6UX46-uniprot.txt
NEW
Summary: ALKAL2 is the ligand that initiates ALK/LTK receptor tyrosine kinase signaling.
Reason: The donkey GOA record has only ERK cascade regulation terms for the process aspect. The ligand does part of the work of this pathway, binding and dimerizing the receptor to start signaling, and human ALKAL2 carries this process from direct assays. Transferred by orthology given the conserved augmentor domain.
We investigated ALK activation by FAM150A and FAM150B proteins in PC12 cell neurite outgrowth assays where we observed a strong activation of ALK signaling.
that FAM150A and FAM150B are potent ligands for human ALK that bind to the
Core Functions
Secreted cytokine ligand that binds the extracellular region of the receptor tyrosine kinases ALK and LTK and activates them by inducing receptor dimerization at the cell surface, initiating downstream MAP kinase signaling.
The cognate cytokines ALKAL1 and ALKAL2 are monomeric three-helix bundles, yet their binding to ALK and LTK elicits similar dimeric assemblies with two-fold symmetry
Q: Does donkey ALKAL2, lacking the primate-specific interchain cysteine, act on ALK as a monomer, and does this change its potency relative to the disulfide-linked human dimer?
Q: Should ALK/LTK ligands (ALKAL1, ALKAL2) be annotated to GO:0030297 transmembrane receptor protein tyrosine kinase activator activity, as for EGF, NGF and VEGFA, rather than GO:0030298 receptor signaling protein tyrosine kinase activator activity, and should the InterPro2GO mapping for IPR029364 be updated accordingly?
Q: Is the ligand activity of ALKAL2 at ALK dependent on co-binding of heparin or other glycosaminoglycans in non-primate mammals, as reported for human ALKAL2-driven ALK activation?
Suggested Experiments
Experiment: Size-exclusion chromatography with multi-angle light scattering of recombinant donkey ALKAL2, alone and with the ALK extracellular region, combined with ALK phosphorylation dose-response assays compared with human ALKAL2.
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.
Assessment of the single ProtNLM2 GO prediction for donkey ALKAL2 (A0A9L0J5U7), judged against the donkey GOA record (extracellular region, plasma membrane, receptor signaling protein tyrosine kinase activator activity, receptor tyrosine kinase binding, ERK1/2 and ERK5 cascade regulation), the human ortholog Q6UX46 and its characterization, and a donkey-human alignment (86% identity, invariant augmentor domain). The prediction, extracellular region, is correct and already in donkey GOA (CNN). The frozen ProtNLM2 output contains no non-GO rows (function text or subcellular location) for this accession.
Prediction method: ProtNLM2 · Version: UniProt API snapshot 2026-10-14
Review rationale: ALKAL2 is a secreted cytokine; the human ortholog is experimentally secreted and active when supplied as conditioned medium, and the donkey protein has a predicted N-terminal signal peptide and is 86% identical to human across its full length. The exact term is already in donkey GOA (UniProt subcellular-location mapping, accepted as a core location in the gene review), so the prediction is correct but not novel. Human ALKAL2 also associates with the cell membrane once bound to ALK, but extracellular region is the appropriate primary location for the ligand.
PMID:26418745: "Addition of conditioned medium from cells expressing FAM150A or FAM150B led to strong activation of ALK in receiving cells that expressed ALK"