Putative function in synaptic vesicle exocytosis by binding to Munc18-1, 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 Munc18-1 and syntaxin-1. Component of the brain-specific heterotrimeric complex (LIN-10-LIN-2-LIN-7 complex) composed of at least APBA1, CASK, and LIN7, which associates with the motor protein KIF17 to transport vesicles along microtubules (By similarity). Within the complex, interacts (via PDZ domain) with the motor protein KIF17; the interaction is direct and is required for association of KIF17 with the cargo that is to be transported (By similarity). Binds to the cytoplasmic domain of amyloid protein (APP) (By similarity). Interacts (via PDZ 1 and 2 domains) with FSPB (By similarity). Isoform 2 interacts (via its truncated PID domain) with active, GTP-bound RAB6A and RAB6B (By similarity). Only a small proportion of the protein is nuclear. Brain. Detected in the cerebellum, hippocampus, olfactory system, piriform and entorhinal cortex, supraoptic nucleus of the hypothalamus, substantia nigra, and other mesencephalic areas. Composed of an N-terminal domain that binds Munc18-1 and LIN-2/CASK, 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. The autoinhibitory helix linker occludes the APP binding site. The PID domain, truncated by 11 amino acids, as observed in isoform 2, but not full-length, mediates the interaction with RAB6A. This isoform interacts with RAB6 GTPases.