Insulin protofilaments

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). 

Jimenez et al. (2002) proposed a molecular model for the insulin protofilament based on these data and on electron cryomicroscopy (cryo-EM) reconstructions of insulin fibrils. The fibrils show a number of twisted morphologies that seem to be alternative packings of similar protofilaments. The protofilaments have cross sections of 30x40 A. The authors suggest a complete conversion to / -structure and model the amyloid monomer as having four jS-strands (Fig. 3B). Each insulin chain contributes two of these jS-strands, and the chains align in a parallel stack, constrained by the interchain disulfide bonds. One pair of stacked /i-stran ds is curved... 

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.

It is common among fibril-forming proteins that a single protein can form fibrils of varied morphology. Variations may arise at the level of subunit conformation, which seems to be the case for the prion proteins, or at the level of protofilament packing, as described in the preceding for insulin. 

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