Role of the C-Terminal 28 Residues of â2-Microglobulin in Amyloid Fibril Formation†

ABSTRACT: â2microglobulin (â2m) is the major protein component of the fibrillar amyloid deposits isolated from patients diagnosed with dialysis-related amyloidosis (DRA). While investigating the molecular mechanism of amyloid fibril formation by â2m, we found that the â2m C-terminal peptide of 28 residues (câ2m) itself forms amyloid fibrils. When viewed by electron microscopy, câ2m aggregates appear as elongated unbranched fibers, the morphology typical for amyloids. Câ2m fibers stain with Congo red and show apple-green birefringence in polarized light, characteristic of amyloids. The observation that the â2m C-terminal fragment readily forms amyloid fibrils implies that â2m amyloid fibril formation proceeds via interactions of amyloid forming segments, which become exposed when the â2m subunit is partially unfolded. Amyloidosis is the process of protein aggregation associ-ated with a variety of human degenerative diseases (1). These pathological amyloid aggregates consist of elongated fibers, resistant to disruption. The components of any given amyloid fibril are primarily of a single-type protein and may form from wild-type, mutant, or truncated proteins. Similar fibrils can be formed in vitro from oligopeptides and denatured proteins (2-4). â2-microglobulin (â2m)1 has been used as a model system to study the mechanism of amyloid fibril formation (5-8). Normally, â2m is the light chain of the type I major histocompatability complex responsible for the presentation of peptides to the immune system. In its native state, â2m adopts a typical immunoglobulin fold consisting of seven â-strands organized into two â-sheets connected by a single disulfide bridge (9) (Figure 1a). In addition, â2m has a pathological role. It was discovered to be the major com-ponent of the amyloid deposits taken from patients diagnosed with dialysis-related amyloidosis (DRA), a serious complica-tion of long-term hemodialysis (10, 11). Different segments of â2m were isolated from ex vivo amyloid fibrils consisting of â2m truncated at residues Ile7