The three-dimensional structure of human β-endorphin amyloid fibrils

Abstract

In the pituitary gland, hormones are stored in a functional amyloid state within acidic secretory granules before they are released into the blood. To gain a detailed understanding of the structure–function relationship of amyloids in hormone secretion, the three-dimensional (3D) structure of the amyloid fibril of the human hormone β-endorphin was determined by solid-state NMR. We find that β-endorphin fibrils are in a β-solenoid conformation with a protonated glutamate residue in their fibrillar core. During exocytosis of the hormone amyloid the pH increases from acidic in the secretory granule to neutral level in the blood, thus it is suggested—and supported with mutagenesis data—that the pH change in the cellular milieu acts through the deprotonation of glutamate 8 to release the hormone from the amyloid. For amyloid disassembly in the blood, it is proposed that the pH change acts together with a buffer composition change and hormone dilution. In the pituitary gland, peptide hormones can be stored as amyloid fibrils within acidic secretory granules before release into the blood stream. Here, we use solid-state NMR to determine the 3D structure of the amyloid fiber formed by the human hormone β-endorphin. We find that β-endorphin fibrils are in a β-solenoid conformation that is generally reminiscent of other functional amyloids. In the β-endorphin amyloid, every layer of the β-solenoid is composed of a single peptide and protonated Glu8 is located in the fibrillar core. The secretory granule has an acidic pH but, on exocytosis, the β-endorphin fibril would encounter neutral pH conditions (pH 7.4) in the blood; this pH change would result in deprotonation of Glu8 to release the hormone peptide from the amyloid. Analyses of β-endorphin variants carrying mutations in Glu8 support the role of the protonation state of this residue in fibril disassembly, among other environmental changes.