Human beta two microglobulin: Difference between revisions
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==Mhb2m fiber formation mechanism at neutral pH== | ==Mhb2m fiber formation mechanism at neutral pH== | ||
===A structural trigger=== | ===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 | 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 ( | ||
<scene name='Human_beta_two_microglobulin/P32a/1'>P32A</scene>, P32G, D59P) of the protein all enhance its ability to form amyloid in vitro, 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. | |||
The b2m variants, P32G and P32V, have been used to show that wild-type b2m populates a native-like folding intermediate (called | |||
IT) en route to the native state that contains a non-native trans Pro32 bond. Increased population of this intermediate in the variant P32G was shown to be concomitant with increased ability of the protein to elongate b2m fibril seeds. Other variants, such as P5G and DN6, also affect isomerisation of the Pro32 peptide bond, promoting population of IT and enabling fibril nucleation at pH 7.0. | |||
In separate studies b2m has been shown to form oligomers and fibrils at neutral pH by addition of Cu2+ and 1 M urea. This induces the formation of a non-native species, called M*, that appears identical to IT by size exclusion chromatography (SEC). Coordination of the metal ion promotes peptide bond isomerisation at Pro32, and the subsequent rapid formation of oligomers as | |||
judged by SEC, providing further supporting evidence that the isomerisation of Pro32 is a key initial step on the pathway to fibrils.Structural studies of the trapped folding intermediate, IT (orM*), have provided molecular insights into the possible aggregation mechanisms of b2m at neutral pH. For example, crystallographic data for the mutant, P32A shows that this conformer results in substantial reorganisation of aromatic side-chains that are present in the core of the native protein. In this structure, a number of aromatic and hydrophobic residues are displaced including Phe30, Phe56, Trp60, Phe62, Tyr63 and Leu54. These rearrangements present a strip of exposed hydrophobic residues on the protein surface, providing a possible avenue for protein aggregation. | |||
[[Image:Conformation change of P32A.png]] | |||
Fig.3. summary of hydrophobic side chain differences resulting from a ''trans'' backbone at position 32 | |||
