Search

Light chain (AL) amyloidosis is characterized by deposition of immunoglobulin light chains (LC) as fibrils in target organs. Alongside the full-length protein, abundant LC fragments are always present in AL deposits. Herein, by combining gel-based and mass spectrometry analyses, we identified and compared the fragmentation sites of amyloid LCs from multiple organs of an AL λ amyloidosis patient (AL-55). The positions pinpointed here in kidney and subcutaneous fat, alongside those previously detected in heart of the same patient, were aligned and mapped on the LC's dimeric and fibrillar states. All tissues contain fragmented LCs along with the full-length protein; the fragment pattern is coincident across organs, although microheterogeneity exists. Multiple cleavage positions were detected; some are shared, whereas some are organ-specific, likely due to a complex of proteases. Cleavage sites are concentrated in "proteolysis-prone" regions, common to all tissues. Several proteolytic sites are not accessible on native dimers, while they are compatible with fibrils. Overall, data suggest that the heterogeneous ensemble of LC fragments originates in tissues, and is consistent with digestion of preformed fibrils, or with the hypothesis that initial proteolytic cleavage of the constant domain triggers the amyloidogenic potential of LCs, followed by subsequent proteolytic degradation. This work provides a unique set of molecular data on proteolysis from ex vivo amyloid, which allows discussing hypotheses on role and timing of proteolytic events occurring along amyloid formation and accumulation in AL patients.

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.

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.

β2 microglobulin (β2m) is the light chain of class-I major histocompatibility complex (MHC-I). Its accumulation in the blood of patients affected by kidney failure leads to amyloid deposition around skeletal joints and bones, a severe condition known as Dialysis Related Amyloidosis (DRA). In an effort to dissect the structural determinants of β2m aggregation, several β2m mutants have been previously studied. Among these, three single-residue mutations in the loop connecting strands D and E (W60G, W60V, D59P) have been shown to affect β2m amyloidogenic properties, and are here considered. To investigate the biochemical and biophysical properties of wild-type (w.t.) β2m and the three mutants, we explored thermal unfolding by Trp fluorescence and circular dichroism (CD). The W60G mutant reveals a pronounced increase in conformational stability. Protein oligomerization and reduction kinetics were investigated by electrospray-ionization mass spectrometry (ESI-MS). All the mutations analyzed here reduce the protein propensity to form soluble oligomers, suggesting a role for the DE-loop in intermolecular interactions. A partially folded intermediate, which may be involved in protein aggregation induced by acids, accumulates for all the tested proteins at pH 2.5 under oxidizing conditions. Moreover, the kinetics of disulfide reduction reveals specific differences among the tested mutants. Thus, β2m DE-loop mutations display long-range effects, affecting stability and structural properties of the native protein and its low-pH intermediate. The evidence presented here hints to a crucial role played by the DE-loop in determining the overall properties of native and partially folded β2m.

The thiamin diphosphate-dependent enzyme indolepyruvate decarboxylase catalyses the formation of indoleacetaldehyde from indolepyruvate, one step in the indolepyruvate pathway of biosynthesis of the plant hormone indole-3-acetic acid. The crystal structure of this enzyme from Enterobacter cloacae has been determined at 2.65 A resolution and refined to a crystallographic R-factor of 20.5% (Rfree 23.6%). The subunit of indolepyruvate decarboxylase contains three domains of open α/β topology, which are similar in structure to that of pyruvate decarboxylase. The tetramer has pseudo 222 symmetry and can be described as a dimer of dimers. It resembles the tetramer of pyruvate decarboxylase from Zymomonas mobilis, but with a relative difference of 20° in the angle between the two dimers. Active site residues are highly conserved in indolepyruvate/pyruvate decarboxylase, suggesting that the interactions with the cofactor thiamin diphosphate and the catalytic mechanisms are very similar. The substrate binding site in indolepyruvate decarboxylase contains a large hydrophobic pocket which can accommodate the bulky indole moiety of the substrate. In pyruvate decarboxylases this pocket is smaller in size and allows discrimination of larger vs. smaller substrates. In most pyruvate decarboxylases, restriction of cavity size is due to replacement of residues at three positions by large, hydrophobic amino acids such as tyrosine or tryptophan.