Crystal structure of monomeric human beta-2-microglobulin reveals clues to its amyloidogenic properties

Abstract

Dissociation of human beta-2-microglobulin (beta(2)m) from the heavy chain of the class I HLA complex is a critical first step in the formation of amyloid fibrils from this protein. As a consequence of renal failure, the concentration of circulating monomeric beta(2)m increases, ultimately leading to deposition of the protein into amyloid fibrils and development of the disorder, dialysis-related amyloidosis. Here we present the crystal structure of a monomeric form of human beta(2)m determined at 1.8-A resolution that reveals remarkable structural changes relative to the HLA-bound protein. These involve the restructuring of a beta bulge that separates two short beta strands to form a new six-residue beta strand at one edge of this beta sandwich protein. These structural changes remove key features proposed to have evolved to protect beta sheet proteins from aggregation [Richardson, J. & Richardson, D. (2002) Proc. Natl. Acad. Sci. USA 99, 2754-2759] and replaces them with an aggregation-competent surface. In combination with solution studies using (1)H NMR, we show that the crystal structure presented here represents a rare species in solution that could provide important clues about the mechanism of amyloid formation from the normally highly soluble native protein.

Figure 1

Ribbon diagram of the crystal structures of (a) M_Hβ₂m and (c) HLAβ₂m. Detailed views of the conformation of residues 49–68 are also shown for (b) M_Hβ₂m and (d) HLAβ₂m. The structure of HLAβ₂m was taken from PDB ID code 1DUZ (25). Individual β strands are labeled A although G. a and c were drawn by using the program MOLSCRIPT (37) and RASTER 3D (38) and b and d, by using SPOCK (39).

Figure 2

Topology diagram of the structures of (a) M_Hβ₂m and (b) HLAβ₂m showing the location of individual β strands in each structure together with their hydrogen-bonding networks. Solid straight line depicts residues in which both the amide nitrogen and carbonyl oxygen form main-chain–main-chain hydrogen bonds; dotted line, residues that make a main-chain–main-chain hydrogen bond involving either the amide nitrogen or the carbonyl oxygen. Data for b were taken from PDB ID code 1DUZ (25). Strands and hydrogen bonds were determined by using the criteria of Kabsch and Sander (40).

Figure 3

Schematic diagram showing the pattern of NH-NH and CαH-CαH NOEs predicted for conformations of M_Hβ₂m (a) lacking and (b) containing the β bulge involving residues 53 and 54. CαH-CαH region (c) and NH-NH region (d) of a ¹H-¹H NOESY spectrum of M_Hβ₂m acquired at pH 5.7, 37°C, in D₂O and H₂O, respectively. Crosspeaks consistent with the presence of the β bulge in solution are marked in bold and underlined. Crosspeaks expected were M_Hβ₂m to adopt a conformation in solution identical to that in a are shown as solid circles.

Figure 4

Ribbon diagram showing the position of His-51 in the crystal structures of (a) M_Hβ₂m and (b) HLAβ₂m. Note that in a, His-51 is outwardly pointing, whereas in b, this residue points inward. Drawn by using SPOCK (39).