Triple /3-spirals

Mitraki and colleagues also overview the other distinctive family of /3-fibrous folds, called the triple /3-spirals (Fig. 3D). The /3-spiral folds are more complicated than the solenoidal fold, with long central /3-strands that hold the trimer together through interchain hydrogen bonds, and... 

RASCHIG RING LESSING RING CROSS-PARTITION RING TRIPLE SPIRAL RING DOUBLE SPIRAL RING... 

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. ... 

Fig. 3. Structure and sequence of repeats present in the fibrous proteins discussed in this chapter. (A) The adenovirus triple -spiral. A single repeat of one of the chains is shown as a stick model colored by atom, the other two as a secondary structure cartoon in yellow and orange. Amino acids contributing to the hydrophobic core are labeled, as is the glycine in the turn. (B) Triple -spiral sequence repeats. Conserved hydrophobic residues are indicated by a hash sign, the conserved glycine or proline by an asterisk. (C) The T4-hber fold. A single repeat of one of the chains is shown as a stick model colored by atom, the other two as a secondary structure cartoon in yellow and orange. Several of the conserved amino acids are labeled. (D) Repeating sequences present in bacteriophage T4 fiber proteins (Cerritelli et al., 1996). Conserved amino acids are indicated by a small letter conserved hydrophobic residues by a hash sign, and conserved small amino acids by a dot.

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).

More recently the crystal structures of the adenovirus fiber shaft and receptor-binding fiber head (van Raaij et al, 1999) and of the head in complex with the coxsackie-adenovirus receptor molecule (GAR) (Bewley et al, 1999) have been solved. The fiber shaft structure revealed a novel (3-sheet triple spiral fold, which is perhaps particularly suited to forming rigid protein projections for interaction with and penetration through cell membranes by adenovirus the structure of the complex that performs the same function in bacteriophage T4 has been solved, revealing a similar fold (Kanamaru et al, 2002). 

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.

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). 

Papanikolopoulou, K., Teixeira, S., Belrhali, H., Forsyth, V. T., Mitraki, A., and van Raaij, M.J. (2004b). Adenovirus fibre shaft sequences fold into the native triple beta-spiral fold when N-terminally fused to the bacteriophage T4 fibritin foldon trimerisation motif. / Mol. Biol. 342, 219-227. 

This patent contains a description of a screw mixer with eccentric, triple-flighted spirals. 

---