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. Component of the LIN-10-LIN-2-LIN-7 complex, which associates with the motor protein KIF17 to transport vesicles containing N-methyl-D-aspartate (NMDA) receptor subunit NR2B along microtubules (By similarity). Part of a multimeric complex containing Munc18-1 and syntaxin-1. Also part of the brain-specific heterotrimeric complex LIN-10/X11-alpha, LIN-2/CASK, and LIN7. 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). Both isoform 1 and isoform 2 bind to the cytoplasmic domain of amyloid protein (APP). Interacts (via PDZ 1 and 2 domains) with FSPB. Isoform 2, but not isoform 1, interacts (via its truncated PID domain) with active, GTP-bound RAB6A and RAB6B. Only about 5% of the protein is located in the nucleus. Brain and spinal cord. Isoform 2 is expressed in testis and brain, but not detected in lung, liver or spleen. 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 and RAB6B. This isoform interacts with RAB6 GTPases.