Putative function in synaptic vesicle exocytosis by binding to STXBP1, an essential component of the synaptic vesicle exocytotic machinery. May modulate processing of the amyloid-beta precursor protein (APP) and hence formation of APP-beta. Part of a multimeric complex containing STXBP1 and syntaxin-1. Binds to the cytoplasmic domain of amyloid-beta protein, and to the nuclear factor NF-kappa-B/p65 via its PDZ domain. Interacts with the N-terminal domain of NECAB3 (By similarity). Specifically expressed in neurons, predominantly of the cerebellum, hippocampus, and spinal cord. Lesser extent in neurons of the cerebral cortex and anterior thalmic nuclei. Composed of an N-terminal domain that binds STXBP1, a middle phosphotyrosine-binding domain (PID/PTB) that mediates binding with the cytoplasmic domain of the amyloid-beta precursor protein, and two C-terminal PDZ domains thought to attach proteins to the plasma membrane. Was originally thought to be the ortholog of human X11 (APBA1).