Figure 6: Tilt angle of the needle filament.

(a) Side view of the hybrid needle structure coloured according to subunit number. The secondary structure of subunit (i) is presented on the left. Blue lines connect successive subunits (i, i+1, i+2, i+3,...) and form a right-handed spiral. The helical axis of the filament is indicated as a bold black line. The red line connects subunits that share an axial interface (i, i+11, i+22, i+33,...). Subunit i+11 (pink) is not directly above subunit (i) as there are N=11.2 protein subunits per two turns of the helix (blue line). As a result, the filament has a slight tilt and the red line forms a slowly rotating left-handed spiral. (b) Schematic representation of the needle filament and tilt angle. The tilt angle (red) is positive in the counter-clockwise direction and can be calculated by α=(360°/N) × (11−N), where N is the number of subunits per two turns. The axial translation between subunit (i) and subunit (i+11) is indicated (2 × L).