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Azinas S.,University of Milan | Azinas S.,University of Surrey | Colombo M.,University of Milan | Barbiroli A.,University of Milan | And 11 more authors.
FEBS Journal | Year: 2011

Proteins hosting main β-sheets adopt specific strategies to avoid intermolecular interactions leading to aggregation and amyloid deposition. Human beta-2 microglobulin (β2m) displays a typical immunoglobulin fold and is known to be amyloidogenic in vivo. Upon severe kidney deficiency, β2m accumulates in the bloodstream, triggering, over the years, pathological deposition of large amyloid aggregates in joints and bones. A β-bulge observed on the edge D β-strand of some β2m crystal structures has been suggested to be crucial in protecting the protein from amyloid aggregation. Conversely, a straight D-strand, observed in different crystal structures of monomeric β2m, could promote amyloid aggregation. More recently, the different conformations observed for the β2m D-strand have been interpreted as the result of intrinsic flexibility, rather than being assigned to a functional protective role against aggregation. To shed light on such contrasting picture, the mutation Asp53→Pro was engineered in β2m, aiming to impair the formation of a regular/straight D-strand. Such a mutant was characterized structurally and biophysically by CD, X-ray crystallography and MS, in addition to an assessment of its amyloid aggregation trends in vitro. The results reported in the present study highlight the conformational plasticity of the edge D-strand, and show that even perturbing the D-strand structure through a Pro residue has only marginal effects on protecting β2m from amyloid aggregation in vitro. Database -Atomic coordinates and structure factors have been deposited in the Protein Data Bank under the accession number. Structured digital abstract to by to by (View Interaction,) to by Beta-2 microglobulin (β2m) is a human amyloidogenic protein. A β-bulge at residue Asp53 within the D β-strand was suggested to be protective against aggregation. We designed the Asp53Pro β2m mutant that hosts a constitutively irregular D-strand, whose conformation is shown to depend on the environment. We propose that regularization of the D-strand is not a requirement for β2m amyloid aggregation. © 2011 FEBS.

Colombo M.,University of Milan | De Rosa M.,University of Milan | Bellotti V.,University of Pavia | Bellotti V.,Laboratori Of Biotecnologie | And 3 more authors.
FEBS Journal | Year: 2012

β-2 microglobulin (β2m) is an amyloidogenic protein responsible for dialysis-related amyloidosis in man. In the early stages of amyloid fibril formation, β2m associates into dimers and higher oligomers, although the structural details of such aggregates are poorly understood. To characterize the protein-protein interactions supporting the formation of oligomers, three individual β2m cysteine mutants and their disulfide-linked homodimers (DIMC20, DIMC50 and DIMC60) were prepared. Amyloid propensity, oligomerization state in solution and crystallogenesis were tested for each β2m homodimer: DIMC20 and DIMC50 display a mixture of tetrameric and dimeric species in solution and also yield protein crystals and amyloid fibrils, whereas DIMC60 is dimeric in solution but does not form protein crystals nor amyloid fibrils. The X-ray structures of DIMC20 and DIMC50 show that the two engineered dimers form a tetrameric assembly; for both tetrameric species, the noncovalent association interface is based on the interaction of facing β2m D-strands and is conserved. Notably, DIMC20 and DIMC50 trigger amyloid formation in wild-type β2m in unseeded reactions. Thus, when the D-D-strand interface is impaired by an intermolecular disulfide bond (as in DIMC60), the formation of tetramers is hindered, and the protein is not amyloidogenic and does not promote amyloid aggregation of wild-type β2m. Implications for β2m oligomerization are discussed. © 2012 FEBS.

Colombo M.,University of Milan | Ricagno S.,University of Milan | Barbiroli A.,University of Milan | Santambrogio C.,University of Milan Bicocca | And 9 more authors.
Journal of Biochemistry | Year: 2011

Beta-2 microglobulin (β2m) is the light chain of Class I major histocompatibility complex (MHC-I) complex. β2m is an intrinsically amyloidogenic protein capable of forming amyloid fibrils in vitro and in vivo. β2m displays the typical immunoglobulin-like fold with a disulphide bridge (Cys25-Cys80) cross-linking the two β-sheets. Engineering of the loop comprised between β-strands D and E has shown that mutations in this region affect protein structure, fold stability, folding kinetics and amyloid aggregation properties. Such overall effects have been related to the DE loop backbone structure, which presents a strained conformation in the wild-type (wt) protein, and a type I β-turn in the W60G mutant. Here, we report a biophysical and structural characterization of the K58P-W60G β2m mutant, where a Pro residue has been introduced in the type I β-turn i + 1 position. The K58P-W60G mutant shows improved chemical and temperature stability and faster folding relative to wt β2m. The crystal structure (1.25 resolution) shows that the Cys25-Cys80 disulphide bridge is unexpectedly severed, in agreement with electrospray ionization-mass spectrometry (ESI-MS) spectra that indicate that a fraction of the purified protein lacks the internal disulphide bond. These observations suggest a stabilizing role for Pro58, and stress a crucial role for the DE loop in determining β2m biophysical properties. © The Authors 2011. Published by Oxford University Press on behalf of the Japanese Biochemical Society. All rights reserved.

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