Amyloid precursor protein at cell membrane, illustration. The amyloid precursor protein (APP, centre) is a complex protein with many functions. It is found on the surface of cells throughout the body. The intact protein binds to many stuctural proteins outside cells, such as heparin and laminin, and sends signals through the G-protein system. When cleaved by enzymes (see F022/1913), one of the products is the beta amyloid peptide that is implicated in conditions such as Alzheimer's disease.