Related Experiment Video
Updated: Aug 15, 2026

Precise Visualization of Insulin Receptors A and B in Murine Brain with an RNA In Situ Hybridization Assay
Published on: July 15, 2025
Proinsulin is refractory to protein fibrillation: topological protection of a precursor protein from cross-beta
Kun Huang1, Jian Dong, Nelson B Phillips
1Department of Biochemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Abstract:
Insulin is susceptible to fibrillation, a misfolding process leading to well ordered cross-beta assembly. Protection from fibrillation in beta cells is provided by sequestration of the susceptible monomer within zinc hexamers. We demonstrate that proinsulin is refractory to fibrillation under conditions that promote the rapid fibrillation of zinc-free insulin. Proinsulin fibrils, as probed by Raman microscopy, are nonetheless similar in structure to insulin fibrils. The connecting peptide, although not well ordered in native proinsulin, participates in a fibril-specific beta-sheet. Native insulin and proinsulin exhibit similar free energies of unfolding as inferred from guanidine denaturation studies: relative amyloidogenicities are thus not correlated with global stability. Strikingly, the susceptibility of proinsulin to fibrillation is increased by scission of the connecting peptide at single sites. We thus propose that the connecting peptide constrains a large scale conformational change in the misfolded protein. A tethering mechanism is proposed based on a model of an insulin protofilament derived from electron-microscopic image reconstruction. The proposed relationship between cross-beta assembly and protein topology is supported by studies of single-chain analogs (mini-proinsulin and insulin-like growth factor I) in which foreshortened connecting peptides further retard fibrillation. In addition to its classic function to facilitate disulfide pairing, the connecting peptide may protect beta cells from toxic protein misfolding in the endoplasmic reticulum.
Related Concept Videos
Insulin: Biosynthesis, Chemistry, and Preparation
Damage or functional impairment of β-cells inhibits insulin production, leading to diabetes. Diabetes treatment primarily uses...
Production of Pharmaceuticals
Protein and Protein Structure
A protein's shape is critical to its function. For example, an enzyme can...
Amyloid Fibrils
Amyloid deposits were observed as early as 1639 in the liver and the spleen. In 1854, Rudolph Virchow performed iodine staining, normally used to...
Insulin Secretory Vesicles
Protein Folding
Protein Structure Is Critical to Its Biological Function
Proteins perform a wide range of biological functions such as catalyzing chemical reactions, providing...

