Polyproline loop

The P3 tyrosine residue of the mouse renin inhibitor complex showing the unique polyproline loop on the right. Specificity of this and neighboring subsites in renins must derive partly from this rigid loop... 

Figure 7. Stereoview of the human renin X-ray crystal structure bound with an inhibitor [43]. The inhibitor is shown in yellow and the enzyme in red with the catalytic aspartic acids, the polyproline loop and Prolll on helix his,2 colored cyan.

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... 

The p,-ARs contain polyproline motifs within their intracellular domains, which in other proteins are known to mediate interactions with SH3 domains. Using the proline-rich third intracellular loop of the (3 r AR as bait, T ang et al. (41) identified SH3p4/p8/pl3 (also referred to as endophilin 1/2/3), a SH3 domain-containing protein family, as binding partners for (3rARs. Both in vitro and in... 

Figure 3 Three-dimensional structure of decorin. On the left is shown a picture of the decorin protein core based on the highest resolution crystal structure (PDB accession code IXKU). The N-terminus is at the top. P-strands are shown in yellow, helices (a, 3-10, and polyproline II) in red, loops and turns in green. All other Class I and Class II SLRPs are believed to have very similar secondary and tertiary structures to that shown here for decorin. On the right are two orthogonal views of the decorin dimer. The protein core of the dimer is shown as a smooth surface in slate blue. The A-terminal 21 amino acids are shown as a smooth surface in marine blue. Spheres are used to represent the N-hnked oligosaccharides (yellow) and the hexasaccharide (marine blue) connecting the GAG chain to serine residue 4 of the protein core. The protein core dimer structure is derived from the crystal structure (PDB accession code IXEC). The other features shown are based on a combination of small-angle X-ray scattering data and molecular modeling (P.G.S., unpublished work).

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