Neuronal receptor tyrosine kinase that is essentially and transiently expressed in specific regions of the central and peripheral nervous systems and plays an important role in the genesis and differentiation of the nervous system (By similarity). Also acts as a key thinness protein involved in the resistance to weight gain: in hypothalamic neurons, controls energy expenditure acting as a negative regulator of white adipose tissue lipolysis and sympathetic tone to fine-tune energy homeostasis (By similarity). Following activation by ALKAL2 ligand at the cell surface, transduces an extracellular signal into an intracellular response. In contrast, ALKAL1 is not a potent physiological ligand for ALK. Ligand-binding to the extracellular domain induces tyrosine kinase activation, leading to activation of the mitogen-activated protein kinase (MAPK) pathway. Phosphorylates almost exclusively at the first tyrosine of the Y-x-x-x-Y-Y motif. Induces tyrosine phosphorylation of CBL, FRS2, IRS1 and SHC1, as well as of the MAP kinases MAPK1/ERK2 and MAPK3/ERK1. ALK activation may also be regulated by pleiotrophin (PTN) and midkine (MDK). PTN-binding induces MAPK pathway activation, which is important for the anti-apoptotic signaling of PTN and regulation of cell proliferation. MDK-binding induces phosphorylation of the ALK target insulin receptor substrate (IRS1), activates mitogen-activated protein kinases (MAPKs) and PI3-kinase, resulting also in cell proliferation induction. Drives NF-kappa-B activation, probably through IRS1 and the activation of the AKT serine/threonine kinase. Recruitment of IRS1 to activated ALK and the activation of NF-kappa-B are essential for the autocrine growth and survival signaling of MDK (By similarity). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated upon ALKAL2 ligand-binding (By similarity). ALKAL2-driven activation is coupled with heparin-binding (PubMed:25605972). Following ligand-binding, homodimerizes and autophosphorylates, activating its kinase activity (By similarity). Inactivated through dephosphorylation by receptor protein tyrosine phosphatase beta and zeta complex (PTPRB/PTPRZ1) when there is no stimulation by a ligand (By similarity). Homodimer; homodimerizes following heparin- and ligand-binding (PubMed:25605972). Interacts with CBL, IRS1, PIK3R1 and PLCG1. Interacts with FRS2 and SHC1. Interacts with PTN and MDK (By similarity). Membrane attachment is essential for promotion of neuron-like differentiation and cell proliferation arrest through specific activation of the MAP kinase pathway. The EGF-like region drives the cytokine specificity for ALKAL2. The heparin-binding region binds heparin glycosaminoglycan (PubMed:25605972). Heparin-binding is required for ALKAL2-driven activation (By similarity). Phosphorylated at tyrosine residues by autocatalysis, which activates kinase activity. In cells not stimulated by a ligand, receptor protein tyrosine phosphatase beta and zeta complex (PTPRB/PTPRZ1) dephosphorylates ALK at the sites in ALK that are undergoing autophosphorylation through autoactivation.