Human beta two microglobulin
From Proteopedia
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Structure of human beta two microglobulin
Beta two microglubulin in human class I major histocompatibility complex (MHCb2m)
Human β2-Microglobulin is the non-covalently bound light chain of its function is to ensure proper folding and cell-surface expression of MHC-1. The natural turnover of MHC-I gives rise to the release of b2m into plasmatic fluids at ~0.1 um and to its catabolism in the kidney. In case of renal dysfunction, b2m concentration increases up to 60-fold, giving rise to pathogenic accumulation of filamentous structures, displaying the typical properties of amyloid fibrils, principally in the joints and connective tissue.
Monomeric human b2m (Mhb2m)
The first crystal structure of is solved in 2002. The protein is 99 residue in length and has a seven-stranded β sandwich fold typical of the Immunoglobulin superfamily. It is stabilized by a single disulfide bond between Cys-25 and Cys-80, which links the two β sheets.
Structural comparison of MHCb2m and Mhb2m
Image:Human b2m bound to MHC-1 .jpg.jpg Image:Momeric human b2m.png
Fig.1. crystal structures of MHCb2m (left)and Mhb2m (right)
Both of the two strucures adopt seven-stranded β sandwich fold with a short C' β strand located in the loop connecting strands C and D. The most significant difference in the ctrystal structures of Mhb2m and MHCb2m involves residues in β strand D and the succeeding loop. When complexed with the MHC heavy chain, residues 50-56 of MHCb2m form two short β strands that separated by a two residue β bulge. These strands (depicted as D1 and D2 in Fig.1) each forms three main-chain-main-chain hydrogen bonds to the adjacent β strand E. The bulge in MHCb2m effectively twists the edge strand, which facilitate its binding to the surface of the heavy chain. However, this β bulge no longer exits in the crytal strucure of Mhb2m. The conformation of D strand in Mhb2m provides an ideal assembly surface, making this edge-strand pair vulnerable to aggregation. The hydrogen-bonding potential of strand D is satisfied by the formation intermolecular interactionswith adjecent molecules, demonstrating the potential for this region to propagate assembly through edge-strand interactions.
In addition, the changes observed in strand D result in differenr orientations of the side chains of residues 50-54. As a result, His-51 (which points inwards in the structure of MHCb2m) rotates by approximately 180, such that it now points away from the hydrophobic core of the protein. This would remove the second protevtive feature from the edge strand, facilitating further interaction in this region.
Fig.2. Ribbon diagram showing the position of HIs-51 in the crystal structure of Mhb2m (left) and MHCb2m (right)
Amyloid Fibril formation of Mhb2m
Mhb2m fiber formation conditions
While the exact factors that cause b2m fibril formation in vivo are not known, several means exist to generate b2m amyloid fibrils in vitro. b2m amyloid fibrils can be generated under acidic conditions (pH < 3.6), by truncating the first six N-terminal amino acids, by dialyzing the protein into distilled water followed by membrane drying, by mixing the protein with collagen at pH=6.4, by sonicating the protein in the presence of sodium dodecyl sulfate at pH=7.0, and by incubating the full-length protein at physiological conditions in the presence of stoichiometric amounts of Cu(II).The latter method is particularly intriguing and may have relevance to the in vivo process because of the very near physiological conditions used.
Mhb2m fiber formation mechanism at neutral pH
A structural trigger
Alterations of the protein sequence have been used to stimulate the formation of fibrils at neutral pH. Specifically, truncation of six residues from the N-terminal region(DN6), as well as mutation in this region (P5G) or in the B/C or D/E loops (P32A, P32G, D59P) of the protein all enhance its ability to form amyloid in vitro (Fig.3), while substitutions elsewhere in the protein have little effect. These studies have the common feature that they encourage partial unfolding of b2m, allowing the aggregation-prone regions of the polypeptide sequence to be exposed and to participate in intermolecular interactions.
