Probable homophilic and heterophilic cell adhesion molecule involved in long term potentiation at hippocampal excitatory synapses through activation of p38MAPK. May also regulate neurite outgrowth by activating the FGFR1 signaling pathway. May play a role in synaptic plasticity (By similarity). Also acts as a chaperone for ATP2B1; stabilizes ATP2B1 and increases its ATPase activity (PubMed:30190470). Promotes localization of XKR8 at the cell membrane (PubMed:27503893). Interacts with ATP2B1; this interaction stabilizes ATP2B1 and increases ATPase activity; this interaction controls T cell calcium homeostasis following T cell activation (PubMed:30190470). Interacts with XKR8; promoting its localization at the cell membrane (PubMed:27503893). Isoform 1 is ubiquitously expressed. Isoform 2 is expressed in brain cortex and cerebellum (at protein level). Some isoforms lack the first Ig-like domain which may confer homophilic adhesion activity. However, they can bind and activate FGFR1 (By similarity). Extended N-terminus.