May play a role in the regulation of hemostasis. The soluble form may have inhibitory properties towards coagulation factors. May interact with cellular G-protein signaling pathways. May bind to the DNA 5'-GTCACATG-3'(CDEI box). Inhibits trypsin, chymotrypsin, plasmin, factor XIA and plasma and glandular kallikrein (By similarity). Modulates the Cu/Zn nitric oxide-catalyzed autodegradation of GPC1 heparan sulfate side chains in fibroblasts. Interacts with CPEB1. Interacts (via NPXY motif) with DAB2 (via PID domain); the interaction is impaired by tyrosine phosphorylation of the NPXY motif. Interacts (via cytoplasmic domain) with APBB2/FE65L (By similarity). Interacts (via intracellular domain) with APBB3/FE65L2 (By similarity). Belongs to the APP family.