SummaryNumerous studies of amyloid assembly have indicated that partially folded protein species are responsible for initiating aggregation. Despite their importance, the structural and dynamic features of amyloidogenic intermediates and the molecular details of how they cause aggregation remain elusive. Here, we use ΔN6, a truncation variant of the naturally amyloidogenic protein β2-microglobulin (β2m), to determine the solution structure of a nonnative amyloidogenic intermediate at high resolution. The structure of ΔN6 reveals a major repacking of the hydrophobic core to accommodate the nonnative peptidyl-prolyl trans-isomer at Pro32. These structural changes, together with a concomitant pH-dependent enhancement in backbone dynamics on a microsecond-millisecond timescale, give rise to a rare conformer with increased amyloidogenic potential. We further reveal that catalytic amounts of ΔN6 are competent to convert nonamyloidogenic human wild-type β2m (Hβ2m) into a rare amyloidogenic conformation and provide structural evidence for the mechanism by which this conformational conversion occurs.

Dialysis-related amyloidosis (DRA) involves the aggregation of beta(2)-microglobulin (beta(2)m) into amyloid fibrils. Using Congo red and thioflavin-T binding, electron microscopy, and X-ray fiber diffraction, we have determined conditions under which recombinant monomeric beta(2)m spontaneously associates to form fibrils in vitro. Fibrillogenesis is critically dependent on the pH and the ionic strength of the solution, with low pH and high ionic strength favoring fibril formation. The morphology of the fibrils formed varies with the growth conditions. At pH 4 in 0.4 M NaCl the fibrils are approximately 10 nm wide, relatively short (50-200 nm), and curvilinear. By contrast, at pH 1.6 the fibrils formed have the same width and morphology as those formed at pH 4 but extend to more than 600 nm in length. The dependence of fibril growth on ionic strength has allowed the conformational properties of monomeric beta(2)m to be determined under conditions where fibril growth is impaired. Circular dichroism studies show that titration of one or more residues with a pK(a) of 4.7 destabilizes native beta(2)m and generates a partially unfolded species. On average, these molecules retain significant secondary structure and have residual, non-native tertiary structure. They also bind the hydrophobic dye 1-anilinonaphthalene-8-sulfonic acid (ANS), show line broadening in one-dimensional (1)H NMR spectra, and are weakly protected from hydrogen exchange. Further acidification destabilizes this species, generating a second, more highly denatured state that is less fibrillogenic. These data are consistent with a model for beta(2)m fibrillogenesis in vitro involving the association of partially unfolded molecules into ordered fibrillar assemblies.

Myosin 10 contains a region of predicted coiled coil 120 residues long. However, the highly charged nature and pattern of charges in the proximal 36 residues appear incompatible with coiled-coil formation. Circular dichroism, NMR, and analytical ultracentrifugation show that a synthesized peptide containing this region forms a stable single ␣-helix (SAH) domain in solution and does not dimerize to form a coiled coil even at millimolar concentrations. Additionally, electron microscopy of a recombinant myosin 10 containing the motor, the three calmodulin binding domains, and the fulllength predicted coiled coil showed that it was mostly monomeric at physiological protein concentration. In dimers the molecules were joined only at their extreme distal ends, and no coiled-coil tail was visible. Furthermore, the neck lengths of both monomers and dimers were much longer than expected from the number of calmodulin binding domains. In contrast, micrographs of myosin 5 heavy meromyosin obtained under the same conditions clearly showed a coiled-coil tail, and the necks were the predicted length. Thus the predicted coiled coil of myosin 10 forms a novel elongated structure in which the proximal region is a SAH domain and the distal region is a SAH domain (or has an unknown extended structure) that dimerizes only at its end. Sequence comparisons show that similar structures may exist in the predicted coiled-coil domains of myosins 6 and 7a and MyoM and could function to increase the size of the working stroke.Myosins make up a diverse superfamily of motor proteins (1). The human genome alone contains about 40 myosin genes (2). Of these, about one-third are "conventional" myosins (i.e. the well studied myosin 2), and the rest fall into about 10 different classes. The structure, properties and functions of the majority of myosin classes are poorly characterized and have largely been inferred from sequence comparisons rather than direct experiments on purified proteins (1-3).Muscle myosin 2 dimerizes through its ␣-helical coiled-coil tail. Therefore, it is commonly assumed that any myosin will also be dimeric if it contains a region predicted to be coiled coil. This assumption is dependent on the accuracy of coiled-coil prediction programs, such as COILS (4), PAIRCOIL, or MULTICOIL (5), which are also used by protein-fold prediction sites on the Web such as SMART (6).Although myosin 10 contains a region of predicted coiled coil (Fig. 1A), and is predicted to dimerize, this has not been determined experimentally. We noticed that part of the predicted coiled-coil domain of myosin 10 is highly enriched in charged residues (Fig. 1B). The proximal region consisting of 36 residues is particularly enriched in both positively and negatively charged residues, including the a and d positions of the heptad repeat (a-g) that are canonically hydrophobic residues in coiled coils (Fig. 1B). We suspected that this highly charged sequence is unlikely to form a coiled coil (7), suggesting that this part of myosin 10 may not be able to dimerize....

Dissociation of human ␤-2-microglobulin (␤2m) 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 ␤2m 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 ␤2m determined at 1.8-Å resolution that reveals remarkable structural changes relative to the HLA-bound protein.These involve the restructuring of a ␤ bulge that separates two short ␤ strands to form a new six-residue ␤ strand at one edge of this ␤ sandwich protein. Beta-2-microglobulin (␤ 2 m) constitutes the noncovalently bound light chain of the class I human leukocyte antigen (HLA class I). The protein is 99 residues in length and has a seven-stranded ␤ sandwich fold typical of the Ig superfamily (1). In vivo, ␤ 2 m is continuously shed from the surface of cells displaying HLA class 1 molecules into the serum, where it is transported to the kidneys to be degraded and excreted. As a consequence of renal failure, the concentration of ␤ 2 m circulating in the serum increases up to 60-fold (2). Free ␤ 2 m then associates to form amyloid fibrils that typically accumulate in the musculoskeletal system and result in the development of the disorder dialysis-related amyloidosis. The majority of ex vivo ␤ 2 m amyloid is comprised of full-length wild-type ␤ 2 m, although small amounts (Ͻ30%) of modified or truncated forms are found (2-4).As for other amyloidogenic proteins (5-7), mild acidification has been shown to promote formation of amyloid fibrils in vitro from pure ␤ 2 m, both de novo (8-10) and by extension of ex vivo material (11). Under acidic conditions, high yields of amyloidlike fibrils displaying a variety of morphologies form spontaneously in vitro (8-10). Incubation of ␤ 2 m at pH 3.6 results in the formation of a partially unfolded species that assembles into short curly fibrils Ϸ10 nm wide and Ͻ200 nm in length. By contrast, fibrils with a long straight morphology, more reminiscent of ex vivo amyloid, are generated by incubation of aciddenatured forms of the protein at low ionic strengths at pH 2.5 and below, either alone (10) or in the presence of fibrillar seeds (11). Amyloid fibrils of ␤ 2 m have also been generated in vitro at neutral pH by deletion of the N-terminal 6 residues (12); incubation of the full length protein in the presence of Cu 2ϩ ions (13); dialysis of the protein at neutral pH into H 2 O followed by its concentration by drying onto the membrane surface (14); and by the extension of ex vivo material, believed to proceed by the assembly of a rarely populated intermediate onto the ends of existing fibrillar seeds (11, 15). The common theme to emerge from these studies is that conformational rearrangements of the normally highly soluble native protein are required for amyloid fibrils to form.More than 80 crystal structures of h...