Fig. 3.

Portions of cylindrical surface lattices, as in Fig. 2, but now with subunits shown as consisting of three α-helices, representing the α-helical components of the outer-tube moiety of the flagellin subunits. The diagrams are only approximately to scale. The α-helices are shown as sausages of diameter 5 Å and are projected onto the reference cylinder of radius 45 Å. The three α-helices in a subunit are bound together as classical left-handed coiled coils by hydrophobic cores, which are here shown stippled. Within each subunit, helices ND1a (with labels e, a, b, g and q) and CD1 (with labels p, c and d) make a long classical coiled coil, while ND1b makes a short, classical coiled coil with portion ea of ND1a. The distal end of the filament is beyond the top of the picture. For the sake of clarity, the upper end of the long helix CD1 has been cut off, so as not to obscure the 11-start connection between point e at the tip of ND1a of subunit i and point g one-third from the bottom of ND1a of subunit i + 11. Likewise, the lower end of the short α-helix ND1b (not labeled) has been cut off, so as not to obscure the upper end of the bi-stable 5-start connection between the middle third of ND1a of subunit i (portion ab) and the bottom third of CD1 of subunit i + 5 (portion cd): this inter-subunit connection, which makes a right-handed coiled coil, is marked by eight short parallel lines. The broken lines on the left mark the tilt of the 11-start lattice lines. (a and b) Straight L and R filaments, respectively. The differently sheared bi-stable connections can be identified by the orientation of the short parallel lines (cf. Fig. 2). Data from Refs. 9 and 10. Note that the R straight filament is some 1.5% shorter than the L. (c) Part of the n = 2 filament, with two strands of bi-stable connections of type R (marked with red lines) and two protofilaments in the R form. These two protofilaments, of slightly shorter length, will only fit onto the surface of the reference cylinder—or, equivalently, onto a plane as here—if the R-type protofilaments are artificially cut and stretched, as shown. It is the pulling together of the sides of these cuts that imparts curvature to the filament.