Promotes a prolonged MAP-kinase signaling by neurotrophins through activation of a Rap1-dependent mechanism. Provides a docking site for the CRKL-C3G complex, resulting in Rap1-dependent sustained ERK activation. May play an important role in regulating postsynaptic signal transduction through the syntrophin-mediated localization of receptor tyrosine kinases such as EPHA4. In cooperation with SNTA1 can enhance EPHA4-induced JAK/STAT activation. Plays a role in nerve growth factor (NGF)-induced recruitment of RAPGEF2 to late endosomes and neurite outgrowth. May play a role in neurotrophin- and ephrin-mediated neuronal outgrowth and in axon guidance during neural development and in neuronal regeneration. Found in a complex, at least composed of KIDINS220, MAGI2, NTRK1 and RAPGEF2; the complex is mainly formed at late endosomes in a nerve growth factor (NGF)-dependent manner (PubMed:17724123). Interacts with RAPGEF2; the interaction is strengthened after NGF stimulation (PubMed:17724123). Isoform 2 interacts (via C-terminal domain) with MAGI2 isoform 1 (via PDZ domain) (PubMed:17724123). Interacts with NTRK1, NTRK2, NTRK3, ERKL and NGFR (PubMed:11150334, PubMed:15167895, PubMed:15378608). Can form a ternary complex with NGFR and NTRK1 and this complex is affected by the expression levels of KIDINS220/ARMS (PubMed:15378608). An increase in KIDINS220/ARMS expression leads to a decreased association of NGFR and NTRK1 (PubMed:15378608). Interacts (via PDZ-binding motif) with SNTA1 and SNTB2 (via PDZ domains) (PubMed:15939763). Interacts with EPHA4 and PRKD1 (PubMed:10998417, PubMed:15939763). Localized at late endosome before or after nerve growth factor (NGF) stimulation. Expressed in developing nervous system and in highly plastic areas of the adult brain. Also expressed in neuroendocrine cells, where it concentrates at the tip of neurites. Expressed in developing muscle and is concentrated at the neuromuscular junction (NMS). SNTA1 can regulate its localization in the NMS. Expressed in postmitotic neurons during the stage of development in which extensive axon pathfinding is occurring. At embryonic day 14 (E14), expressed in both spinal cord and dorsal root ganglia. Present on the sarcolemma in postnstal day 1 (P1) gastrocnemius muscle. By P8, becomes more concentrated at the junctional sites and by P21, colocalizes with acetylcholine receptor clusters. The transmembrane domain mediates interaction with NTRK1. Tyrosine phosphorylated by NTRK1, NTRK2, EPHB2 and EPHA4. Phosphorylation at Ser-918 is induced by phorbol ester treatment. Phosphorylation by NTRK2 is induced by brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5. Phosphorylation by NTRK1 is induced by nerve growth factor (NGF).