Spiral domains

Fig. 5. The triple //-spiral in the adenovirus fiber shaft. (A) The triple //-spiral domain alone. Shown are amino acids 319-392 of each of the three chains, forming four triple /1-spiral repeats (van Raaij et al, 1999b). (B) Structure of a chimeric adenovirus fiber shaft-fibritin foldon domain protein (Papanikolopoulou et al, 2004b).

The tt-A isotherms for these monolayers indicated homochiral discrimination. Similar curved spiral domains were seen in monolayers of A-pahnitoylaspartic acid. Very striking, curved, condensed-phase domains were observed by BAM in monolayers of V-a-palmitoylthreonine, curving with opposite sense for the two pure enantiomers and showing twin-like structures with one arm curving in each sense for the racemic mixture paper. ... 

The other flexoelectric structure can arise in a very thin planar cholesteric layer at a certain set of material parameters. The instabihty occurs in the form of spiral domains [89] whose handedness depends on the sign of an electric field. While the threshold voltage for the hnear domains discussed above slightly depends on cell thickness, Uth 27rRT/ eii - 633 , the threshold for the spiral domains is proportional to thickness... 

It should be noted that with increasing field, the spiral domains would appear first if d < and (en — 633) < 633. Spiral domains have been observed in experiments [90]. 

Later, Palmer et al. (1986) investigated the influence of the sample purity on domain patterns and the hysteresis in the helical phase in Tb and Ho. The domain patterns were observed by polarized neutron diffraction topography. It was found that the size of the spiral domains in the helical state was very sensitive to the sample purity. For the low purity material the size of the domains was below the resolution limit of the method, and... 

Richmond and collaborators (Figure 12.30). The octamer (Figure 12.29) has surface landmarks that guide the course of the DNA around the octamer 146 bp of B-DNA in a flat, left-handed superhelical conformation make 1.65 turns around the histone core (Figure 12.30), which itself is a protein superhelix consisting of a spiral array of the four histone dimers. Histone 1, a three-domain protein, serves to seal the ends of the DNA turns to the nucleosome core and to organize the additional 40 to 60 bp of DNA that link consecutive nucleo-... 

Usually, TS /1-solenoids represent only parts of larger multidomain proteins. Other trimeric motifs found in these proteins include a-helical coiled coils, TS /1-spirals, trimeric bundles of single-stranded /1-solenoids, and irregular globular structures. Some of these domains may be needed for correct folding of the TS /1-solenoid. 

Fig. 1. Schematic drawings of the viruses discussed in this chapter. (A) An icosahe-dral virus with fiber proteins inserted in its pentameric vertices. The gray box denotes domains with known structures for adenovirus, reovirus, and bacteriophage PRD1, in each case containing the head domain and proximal part of the triple /8-spiral shaft domain. (B) Contractile-tailed bacteriophage T4. T4 contains three different fibrous proteins, fibritin connected to the neck, the long (bent) fibers connected to the base plate, and the short fibers also connected to the base plate. Only two of each of the trimeric fibrous proteins are shown for clarity. The gray box denotes domains with known structure for the T4 short fiber.

The partial structure of the adenovirus type 2 fiber shaft was the first /(-structured fibrous fold characterized at atomic detail. The shaft contains 15-residue sequence pseudo-repeats with an invariant glycine or proline and conserved hydrophobic amino acids (Fig. 3A Green et al., 1983). Unfolding studies led to the identification of a stable domain, consisting of amino acids 319-582, which was subcloned, expressed, and purified (Mitraki et al., 1999). Subsequently, it was crystallized, and its structure solved at 2.4-A resolution (van Raaij et al., 1999b). Its structure revealed a new fold, called the triple /(-spiral. ... 

More recently, triple /1-spiral repeats have been identified in mammalian reovirus type 3 fiber (Chappell et al., 2002 Fig. 4A), avian reovirus fiber (Guardado Calvo et al., 2005 Fig. 4B), and bacteriophage PRD1 P5 protein (Merckel et al., 2005 Fig. 4C). In the latter two cases, it appears that only two repeats are present, just N-terminal to the head domain. Mammalian reovirus fiber contains eight putative triple /1-spiral repeats, of which three were resolved in the crystal structure (Chappell et al., 2002). 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

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