Metal Binding Dictates Conformation and Function of the Amyloid Precursor Protein (APP) E2 Domain

Abstract

The amyloid precursor protein (APP) and its neurotoxic cleavage product Aβ are key players in the development of Alzheimer's disease and appear essential for neuronal development and cell homeostasis in mammals. Proteolytic processing of APP is influenced by metal ions, protein ligands and its oligomerization state. However, the structural basis and functional mechanism of APP regulation are hitherto largely unknown. Here we identified a metal-dependent molecular switch located within the E2 domain of APP containing four evolutionary highly conserved histidine residues. Three X-ray structures of the metal-bound molecule were solved at 2.6-2.0 Å resolution. Using protein crystallographic and biochemical methods, we characterized this novel high-affinity binding site within the E2 domain that binds competitively to copper and zinc at physiological concentrations. Metal-specific coordination spheres induce large conformational changes and enforce distinct structural states, most likely regulating the physiological function of APP and its processing in Alzheimer's disease.

Bioinorganic Chemistry of Alzheimer’s Disease

“…73 Furthermore, a toxic mode of Aβ, while several are known and suggested, has not been found to be causative of AD. Even if apo-Aβ oligomers in principle remain plausible key toxic substances in AD pathogenesis, the underlying causes for impaired Aβ balance, which is now known to be substantially controlled by metal ions both at the APP and Aβ processing levels, 17,139 must clearly be addressed. 10,25,73 The metal ion hypothesis 11,73 was inspired by early suggestions 140 and later observations 10,95,141−143 that AD correlated with dyshomeostasis of metal ions, notably first Fe, and later Zn and Cu.…”

“…139 Two intramolecular metal binding sites and several solvent-exposed sites were observed, with the high-affinity M1 site consisting of four histidines (APP sequence numbers His-313, His-382, His-432, and His-436) binding to Cu(II) in a Jahn−Teller distorted square planar geometry. In case of Zn(II), His-313 is substituted for a water ligand (see Figure 6).…”

The human β-amyloid precursor protein: biomolecular and epigenetic aspects

“…In this regard, it is interesting that APP co-localizes with integrins on the surface of axons and at sites of adhesion (30,31). The E2 domain has also a heparin-binding site (HBD2) (32,33) (Figure 1B) as well as a number of putative metalbinding sites that may hold the E2 domain in a rigid conformation (34). Between the E2 domain and the Aβ region are found two potential N-linked glycosylation sites (CHO) at residues 542 and 571 ( Figure 1B).…”

Iron, Aging, and Neurodegeneration

“…APP has the potential to reduce Cu 2+ to Cu + . The E2 domain of APP has also been suggested to have two metal binding sites [58]. Of these, Zn 2+ is thought to be coordinated between His382, His432, and His436.…”