Triple-stranded /3-solenoids

D, Differences Between Sequences of Single-Stranded and Triple-Stranded (3-Solenoids... 

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. 

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. 

Most known //-solenoids are monomers. Recently, however, several triple-stranded (TS) //-solenoids have been described in which the three chains wind around a common axis. The three monomers are related by... 

Fig. 7. Triple-stranded (TS) /8-solenoids. (A) Ribbon diagram of a fragment of the T4 short tail-fiber gpl2 (van Raaij et al., 2001) and (B) cross-sectional shapes of the TS //-solenoids. Repetitive turn-strand elements of each individual chain of the TS //-solenoids are rendered in different colors.

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... 

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