Studies of the Amyloid Precursor Protein (APP) in Brain: Regulation of APP-Ligand Binding

Abstract

The specific binding of the amyloid precursor protein (APP) to glycosaminoglycans (GAG) suggests that APP is a cell adhesion molecule (CAM) and/or substrate adhesion molecule (SAM). In order to characterize this activity of APP in the brain at the molecular level, we have purified and characterized the major APP species from rat brain. The major isoform isolated was sequenced and found to be APP695. In a solid-phase binding assay, the specificity of this brain-specific APP isoform-GAG interaction was assessed. The binding of APP to the glycosaminoglycan heparin was found to be time dependent and saturable. A strong heparin binding site within a region conserved in rodent and human APP was identified. Saturable binding to heparin through this binding site was found to occur at nanomolar concentrations of APP. This putative high-affinity site was then located within a sequence of 22 amino acids in length corresponding to amino acids 316–337 of APP695. This sequence is encoded by APP exon 9 and the first three codons of exon 10. Since all APP and L-APP isoforms so far described include these exons, the strong heparin binding site is a ubiquitous feature of all APP and L-APP isoforms, strongly suggesting that the brain-specific and neuronal as well as the non-neuronal and peripheral APPs and L-APPs do have CAM- and SAM-like activities.