Triple /3-stranded folds

A recently discovered subset of triple-stranded /l-helices from bacteriophage tail proteins (alternatively termed triple-stranded /1-solenoids ) represents another distinct group of /1-fibrous folds (Fig. 3B). In these structures, three identical chains related by threefold rotational symmetry wind around a common axis. These chains form unusual parallel /1-sheets with no intra- and only intermolecular -structural hydrogen bonding. Kajava and Steven (this volume) survey the distinguishing structural features of the known triple-stranded /1-solenoids, also documenting their notable diversity and differences in comparison to the single-stranded /1-solenoids. 

The abundance, location, stability, and folding of the triple-stranded /3-helices are also reviewed in a chapter by Mitraki, Papanikolopoulou, and van Raaij, which is dedicated to triple /3-stranded fibrous folds in the viral fibers. 

All the above-mentioned proteins have single-stranded folds based on solenoidal windings of one polypeptide chain. Recently, however, several triple-stranded /1-helices (alternatively, triple-stranded /l-solenoids ) have been described in bacteriophage tail proteins (Kanamaru et al., 2002 Smith et al., 2005 Stummeyer et al., 2005 van Raaij et al, 2001). In these structures, three identical chains wind around a common axis and their coils have an axial rise of 14.5 A, that is, 3 x 4.83 A (for details see Sections IV and V.D). In this chapter, triple-stranded /l-solenoids will be abbreviated as TS /l-solenoids, while the term /1-solenoid, if not otherwise qualified, will apply to the predominant group of single-stranded /l-solenoids. 

C. Role of Globular Domains in the Folding of Triple /.-Stranded... 

D. Specific Chaperones Involved in the Folding of Triple p-Stranded Folds... 

Helix bundles. A third peptide chain can be added to a coiled coil to form a triple-stranded bundle.180-183 An example is the glycoprotein laminin found in basement membranes. It consists of three peptide chains which, for -600 residues at their C-terminal ends, form a three-stranded coil with heptad repeats.182184 Numerous proteins are folded into four helical segments that associate as four-helix bundles (Fig. 2-22).185-188 These include electron carriers, hormones, and structural proteins. The four-helix bundle not only is a simple packing arrangement, but also allows interactions between the + and - ends of the macro-dipoles of the helices. 

The Structure of the a-Keratins Was Determined with the Help of Molecular Models The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains Collagen Forms a Unique Triple-Stranded Structure Globular Protein Structures Are Extremely Varied and Require a More Sophisticated Form of Analysis Folding of Globular Proteins Reveals a Hierarchy of Structural Organization... 

At first sight it may seem curious to use in vitro selection to identify RNA or DNA sequences that are able to interact with another nucleic strand. SELEX has been used in at least two cases first, in an attempt to extend the repertoire of triple-stranded structures, and second, to recognize folded RNA structures. Triple... 

Figure 6. Computer generator ORTEP projections of the triple-strand structure corresponding to the x-ray diffraction pattern shown in Figure 3. The pitch of 1.746 nm for each six-fold helix is reduced to 1.746/3 = 0.581 nm by the symmetry related chains, (a) and (b) are different projections normal to the molecular axis and (c) is a projection parallel to the molecular axis.

When the sample is dehydrated the X-ray diffraction pattern obtained is of poorer quality (Fig. 4b) and is similar to that reported for lentinan (8). We suggest that the removal of water causes a twisting of the chains back toward the six-fold triple-stranded model. On annealing, the sample completes this transition (Fig. 4c) by exhibiting a pattern similar to Figure 3. The reflections observed in Figure 4c index on a hexagonal unit cell with dimensions a = b = 1.530 nm, c (fibre axis) = 1.76 nm and the measured densTty Ts 1.52 g/cm . 

A model for the seven-fold triple-stranded structure is shown in Figure 7. 

The X-ray evidence favours a model for the curdlan gel based on triple-stranded molecules. This structure is present in both the gel prepared from aqueous suspension and from alkali solution. The alkali gel contains the proposed seven-fold triple-stranded model (Figs. 7 and 14a) which converts to the more symmetric six-fold triple helix by annealing (Figs. 6 and 14b). This... 

Figure 14. Comparison of triple-stranded models involve (a) seven-fold helices ...

The cylindrically averaged Fourier transform of the sevenfold and six-fold triple-stranded structures are shown in Figure 15. The Fourier transform of the six-fold triple-stranded model illustrates the symmetry of the system by the total absence of intensity on layer lines with index i 3 n, where n is an integer. The Fourier transform of the seven-fold triple-stranded structure shows that in destroying this precise symmetry relationship intensity occurs on all layer lines which are orders of the 2.27 nm spacing. This reinforces the concept of an indigenous triple-stranded structure which is perturbed slightly by the interaction of solvent. 

---