The Intrachain Disulfide Bond of β2-Microglobulin Is Not Essential for the Immunoglobulin Fold at Neutral pH, but Is Essential for Amyloid Fibril Formation at Acidic pH

β2-Microglobulin (β2M), the light chain of the type I major histocompatibility complex, is a major component of dialysis-related amyloid fibrils. β2M in the native state has a typical immunoglobulin fold with a buried intrachain disulfide bond. The conformation and stability of recombinant β2M in which the intrachain disulfide bond was reduced were studied by CD, tryptophan fluorescence, and one-dimensional NMR. The conformation of the reduced β2M in the absence of denaturant at pH 8.5 was similar to that of the intact protein unless the thiol groups were modified. However, reduction of the disulfide bond decreased the stability as measured by denaturation in guanidine hydrochloride. Intact β2M formed amyloid fibrils at pH 2.5 by extension reaction using sonicated amyloid fibrils as seeds. Under the same conditions, reduced β2M did not form typical amyloid fibrils, although it inhibited fibril extension competitively, suggesting that the conformation defined by the disulfide bond is important for amyloid fibril formation of β2M