Amyloid fibril cross-/3 structure

The aggregation of protein fibrils in organs is believed to be the cause of several degenerative disorders including Alzheimer s and Parkinson s disease. Amyloid fibrils are structures which, regardless of the identity of the protein, share a common cross-P-sheet core structure. Several Raman spectroscopic studies have focused on insulin amyloid fibrils (14-19). We have recently used DCDR to confirm the previously reported results and perform a more detailed difference spectroscopic analysis to quantify the principal Raman spectral features associated with insulin fibrillation (20). The following results demonstrate that very similar DCDR difference spectral features are observed upon fibrillation of lysozyme. 

To this list of protein misfolding diseases can be added rare familial amyloidoses in which the mutated proteins have the classic amyloid fibril congophilic birefringence and cross-(3-sheet structure (Table 3). Many of these deposits have an impact on the central nervous system (TTR, cystatin, lysozyme) as well as on other organ systems. A newly described disease, familial British dementia, is associated with the deposition of Abri, a 34 amino acid, 4 kDa peptide cleaved from a 277 amino acid precursor sequence, the last 10 amino acids of which are not normally translated [52]. Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is... 

One aspect of the silk fibril formed in solution remains unclear the apparent absence of the cross-/l structure that characterizes amyloid fibrils. 

A trademark of amyloid fibrils is their cross-/ structure. This structure is the basis of the repetitive hydrogen-bonding extension of the fibril (Makin et al., 2005). Cross-/ structures are observed in the silk fibers of some insects (Geddes et al., 1968 Hepburn et al., 1979), although none are observed in spiders or lepidoptera (Craig, 1997). This absence has been explained by the possibility that cross-/ silks or a-silks may be converted into collinear /1-silks by stretching the fiber and an increased orientation-function correlated to the speed at which silk is formed (Riekel et al., 2000). 

Amyloid fibrils form from a variety of native proteins with diverse sequences and folds. The classic method for the structural analysis of amyloid has been X-ray fiber diffraction amyloid fibrils exhibit a characteristic diffraction signature, called the cross-/) pattern. This cross-/ pattern suggested a repeating structure in which /1-sheets run parallel to the fiber axis with their constituent /1-strands perpendicular to that direction (Sunde and Blake, 1997). This diffraction signature pointed to an underlying common core molecular structure for the amyloid fibril that could accommodate diverse sequences and folds. A number of groups have proposed amyloid folds that are consistent with the experimental data and these can be linked to repeating /1-structured units. 

Fig. 1. Structure of amyloid fibrils formed by the human amylin peptide. Negatively stained (A) and metal shadowed (B) fibrils formed by human amylin (adapted from Goldsbury et al., 2000a). (C) A human amylin fibril model formed by three protofibrils having a superpleated /i-structure (adapted from Kajava et al., 2005). Only Ca traces of the polypeptide chains are shown. (D) Atomic model of the cross-/ motif formed by the human amylin peptide (adapted from Kajava et al, 2005). Scale bar, 100 nm (A and B).

Fig. 1. Cross-/] structure of amyloid fibrils. (A) Cartoon representation of a cross-/] X-ray diffraction pattern. The defining features are a meridional reflection at 4.7 A and an equatorial reflection on the order of 10 A. The 4.7-A reflection is generally much brighter and sharper than the reflection at 10 A. (B) The cross-/] core structure of amyloid fibrils. Parallel /(-sheets are depicted, but the structure could equivalendy be composed of antiparallel /(-sheets or a mix of parallel and antiparallel. The 4.7-A spacing of /(-strands within each /(-sheet is parallel to the long fibril axis. The depicted 10-A sheet-to-sheet spacing actually ranges from about 5 to 14 A (Fandrich and Dobson, 2002), depending on the size and packing of amino acid side chains. Amyloid fibrils have diameters on the order of 100 A.

A prime example of a Refolding model is that of the insulin protofilament (Jimenez et al., 2002). Insulin is a polypeptide hormone composed of two peptide chains of mainly o -helical secondary structure (Fig. 3A Adams et al., 1969). Its chains (21- and 30-amino acids long) are held together by 3 disulfide bonds, 2 interchain and 1 intrachain (Sanger, 1959). These bonds remain intact in the insulin amyloid fibrils of patients with injection amyloidosis (Dische et al., 1988). Fourier transform infrared (FTIR) and circular dichroic (CD) spectroscopy indicate that a conversion to jS-structure accompanies insulin fibril formation (Bouchard et al., 2000). The fibrils also give a cross-jS diffraction pattern (Burke and Rougvie, 1972). 

Fig. 3. Refolding model of insulin protofilaments, from Jimenez et al. (2002). (A) Ribbon diagram of the crystal structure of porcine insulin (PDB ID code 3INS), generated with Pymol (DeLano, 2002). The two chains are shown as dark and light gray with N- and C-termini indicated. The dotted lines represent the three disulfide bonds 1 is the intrachain and 2 and 3 are the interchain bonds. (B) Cartoon representation of the structure of monomeric insulin in the fibril, as proposed by Jimenez et al. (2002). The thick, arrowed lines represent /1-strands, and thinner lines show the remaining sequence. The disulfide bonds are as represented in panel A, and N- and C-termini are indicated. (Components of this panel are not to scale.) (C) Cartoon representation of an insulin protofilament, showing a monomer inside. The monomers are proposed to stack with a slight twist to form two continuous /(-sheets. (Components of this panel, including the protofilament twist, are not to scale.) In the fibril cross sections presented byjimenez et al. (2002), two, four, or six protofilaments are proposed to associate to form the amyloid-like fibrils.

A cross-jS spine model was proposed for the fibril structure of human /]2-microglobulin (h/]2m) (Ivanova et al., 2004). h/I2m is a 99-amino acid serum protein with a 7-stranded /(-sandwich fold (Fig. 10A Saper et al, 1991). In patients on long-term kidney dialysis, the protein is deposited as amyloid fibrils in the joints (Floege and Ehlerding, 1996 Koch, 1992). In vitro-formed fibrils of h/)2m give a cross-/] X-ray diffraction pattern (Ivanova et al., 2004 Smith et al., 200S). Several studies have shown that segments of h/]2m form amyloid-like fibrils on their own (Ivanova et al., 2003 Jones et al., 2003 Kozhukh et al, 2002). 

In this chapter, we present several examples of structural models for amyloid fibrils, which we group into general classes. None of these general model classes can completely explain the common properties of amyloid and amyloid-like fibrils however, the Gain-of-Interaction models with a cross-/ spine seem most consistent with what is known. These models combine the structural aspect of the cross-/ spine with the specificity of sequence-dependent interactions to explain the observed diffraction, stability, and self-only association of amyloid fibrils. It is also possible... 

What is the nature of the insoluble forms of the prion protein They are hard to study because of the extreme insolubility, but the conversion of a helix to (3 sheet seems to be fundamental to the process and has been confirmed for the yeast prion by X-ray diffraction.11 It has been known since the 1950s that many soluble a-helix-rich proteins can be transformed easily into a fibrillar form in which the polypeptide chains are thought to form a P sheet. The chains are probably folded into hairpin loops that form an antiparallel P sheet (see Fig. 2-ll).ii-11 For example, by heating at pH 2 insulin can be converted to fibrils, whose polarized infrared spectrum (Fig. 23-3A) indicates a cross-P structure with strands lying perpendicular to the fibril axis >mm Many other proteins are also able to undergo similar transformation. Most biophysical evidence is consistent with the cross-P structure for the fibrils, which typically have diameters of 7-12 rnn."-11 These may be formed by association of thinner 2 to 5 nm fibrils.00 However, P-helical structures have been proposed for some amyloid fibrils 3 and polyproline II helices for others. 1 11... 

---