K3 fragment of amyloidogenic β₂-microglobulin forms ion channels: Implication for dialysis related amyloidosis

β₂-microglobulin (β₂m) amyloid deposits are linked to dialysis-related amyloidosis (DRA) in hemodialysis patients. The mechanism by which β₂m causes DRA is not understood. It is also unclear whether only the full-length β₂m induces pathophysiology or if proteolytic fragments are sufficient for inducing this effect. Ser20-Lys41 (K3) is a digestion fragment of full-length β₂m. Solid state NMR (ssNMR) combined with X-ray diffraction and atomic force microscopy (AFM) revealed the characteristic oligomeric amyloid conformation of the U-turn β-strand-turn-β-strand motif stacked in parallel and stabilized by intermolecular interactions also shown by Aβ_9-40/ Aβ_17-42 and the CA150 WW domain. Here we use the K3 U-turn atomic coordinates and molecular dynamic (MD) simulations to model K3 channels in the membrane. Consistent with previous AFM imaging of other amyloids that show channel-like structures in the membrane, in the simulations K3 also forms ion channels with 3-6 loosely attached mobile subunits. We carry out AFM, single channel electrical recording, and fluorescence imaging experiments. AFM images display 3D ion channel topography with shapes, morphologies, and dimensions consistent with the theoretical model. Electrical conductance measurements indicate multiple single channel conductances, suggesting that various K3 oligomer sizes can constitute the channel structure. Fluorescence measurements in kidney cells show channel-mediated cell calcium uptake. These results suggest that the β₂m-induced DRA can be mediated by ion channels formed by its K3 fragment. Because the β-strand-turn-β-strand motif appears to be a universal amyloid feature, its ability to form ion channels further suggests that the motif may play a generic role in toxicity.