Functions as a cell surface receptor and performs physiological functions on the surface of neurons relevant to neurite growth, neuronal adhesion and axonogenesis. Interaction between APP molecules on neighboring cells promotes synaptogenesis. Involved in cell mobility and transcription regulation through protein-protein interactions (By similarity). Can promote transcription activation through binding to APBB1-KAT5 and inhibit Notch signaling through interaction with Numb (By similarity). Couples to apoptosis-inducing pathways such as those mediated by G(o) and JIP (By similarity). Inhibits G(o)-alpha ATPase activity (By similarity). Acts as a kinesin I membrane receptor, mediating the axonal transport of beta-secretase and presenilin 1 (By similarity). May be involved in copper homeostasis/oxidative stress through copper ion reduction (By similarity). In vitro, copper-metallated APP induces neuronal death directly or is potentiated through Cu(2+)-mediated low-density lipoprotein oxidation (By similarity). Can regulate neurite outgrowth through binding to components of the extracellular matrix such as heparin and collagen I and IV. Induces a AGER-dependent pathway that involves activation of p38 MAPK, resulting in internalization of amyloid-beta peptide and mitochondrial dysfunction in cultured cortical neurons. Provides Cu(2+) ions for GPC1 which are required for release of nitric oxide (NO) and subsequent degradation of the heparan sulfate chains on GPC1 (By similarity). Binds, via its C-terminus, to the PID domain of several cytoplasmic proteins, including APBB family members, the APBA family, MAPK8IP1, SHC1 and NUMB and DAB1 (By similarity). Binding to DAB1 inhibits its serine phosphorylation (By similarity). Interacts (via NPXY motif) with DAB2 (via PID domain); the interaction is impaired by tyrosine phosphorylation of the NPXY motif. Also interacts with GPCR-like protein BPP, APPBP1, IB1, KNS2 (via its TPR domains), APPBP2 (via BaSS) and DDB1. In vitro, it binds MAPT via the MT-binding domains (By similarity). Associates with microtubules in the presence of ATP and in a kinesin-dependent manner (By similarity). Interacts, through a C-terminal domain, with GNAO1. Interacts with CPEB1, ANKS1B, TNFRSF21 and AGER (By similarity). Interacts with ITM2B. Interacts with ITM2C. Interacts with IDE. Can form homodimers; dimerization is enhanced in the presence of Cu(2+) ions. Can form homodimers; this is promoted by heparin binding (By similarity). Interacts with SORL1 (via N-terminal ectodomain); this interaction retains APP in the trans-Golgi network and reduces processing into soluble APP-alpha and amyloid-beta peptides (By similarity). Interacts with PLD3 (By similarity). Interacts with VDAC1 (By similarity). Interacts with NSG1; could regulate APP processing (By similarity). Interacts with LRRK2 (By similarity). Interacts (via cytoplasmic domain) with KIF5B (By similarity). Interacts (via C-terminus) with APBB2/FE65L1 (via C-terminus) (By similarity). Interacts (via intracellular domain) with APBB3 (By similarity). Cell surface protein that rapidly becomes internalized via clathrin-coated pits. Only a minor proportion is present at the cell membrane; most of the protein is present in intracellular vesicles. During maturation, the immature APP (N-glycosylated in the endoplasmic reticulum) moves to the Golgi complex where complete maturation occurs (O-glycosylated and sulfated). After alpha-secretase cleavage, soluble APP is released into the extracellular space and the C-terminal is internalized to endosomes and lysosomes. Some APP accumulates in secretory transport vesicles leaving the late Golgi compartment and returns to the cell surface. Proteolytically processed under normal cellular conditions. Cleavage either by alpha-secretase, beta-secretase or theta-secretase leads to generation and extracellular release of soluble APP peptides, S-APP-alpha and S-APP-beta, and the retention of corresponding membrane-anchored C-terminal fragments, C80, C83 and C99. Subsequent processing of C80 and C83 by gamma-secretase yields P3 peptides. This is the major secretory pathway and is non-amyloidogenic. Alternatively, presenilin/nicastrin-mediated gamma-secretase processing of C99 releases the amyloid-beta proteins, amyloid-beta protein 40 and amyloid-beta protein 42, major components of amyloid plaques, and the cytotoxic C-terminal fragments, gamma-CTF(50), gamma-CTF(57) and gamma-CTF(59). PSEN1 cleavage is more efficient with C83 than with C99 as substrate (in vitro). Amyloid-beta protein 40 and Amyloid-beta protein 42 are cleaved by ACE. Many other minor amyloid-beta peptides, amyloid-beta 1-X peptides, are found in cerebral spinal fluid (CSF) including the amyloid-beta X-15 peptides, produced from the cleavage by alpha-secretase. Belongs to the APP family.