Postsynaptic adhesion molecule that binds to presynaptic neurexins to mediate both excitatory and inhibitory synapse formation (By similarity). Promotes synapse development by acting as a cell adhesion molecule at the postsynaptic membrane, which associates with neurexin-alpha at the presynaptic membrane (By similarity). Also functions as a cargo in axonal anterograde transport by acting as a molecular adapter that promotes KLC1 association with vesicles (PubMed:21385839). Complex formation with APBA2 and APP, stabilizes APP metabolism and enhances APBA2-mediated suppression of beta-APP40 secretion, due to the retardation of intracellular APP maturation (PubMed:12972431). {ECO:0000250|UniProtKB:Q99JH7, ECO:0000250|UniProtKB:Q9EPL2, ECO:0000269|PubMed:12972431, ECO:0000269|PubMed:21385839}. [Soluble Alc-alpha]: As intracellular fragment AlcICD, suppresses APBB1-dependent transactivation stimulated by APP C-terminal intracellular fragment (AICD), most probably by competing with AICD for APBB1-binding (PubMed:15037614). . [CTF1-alpha]: In complex with APBA2 and C99, a C-terminal APP fragment, abolishes C99 interaction with PSEN1 and thus APP C99 cleavage by gamma-secretase, most probably through stabilization of the direct interaction between APBA2 and APP (PubMed:15037614). . This is the function of Ensembl gene identifier ENSG00000171603 (CLSTN1, calsyntenin 1).