Fig. 4: Screw axis asymmetry in the M. villosus archaellum.
From: An archaellum filament composed of two alternating subunits
a The archaellum atomic model viewed parallel to the filament’s axis. ArlB1 (cyan) and ArlB2 (sand) subunits are highlighted along one left-handed 3-start helical strand. ArlB1 and ArlB2 alternate in the 3-start direction (indicated by the red arrows). b Subunits within all three 3-start strands are highlighted. Every third 3-start strand (red arrow) is out of register with respect to the other two 3-start strands (black arrows). c Intermolecular contacts in 3-start direction between ArlB1-2 (top) and ArlB2-1 (bottom) dimers. The dissociation energies (∆Gdiss calculated by PISA) for both interactions are indicated below each dimer (unit: kcal/mol). d Close-ups of the atomic models (head domains) of ArlB1-2 and ArlB2-1 as in c. e Tracing subunits of the same kind through the filament indicates non-helical homopolymeric pseudo-strands (dashed lines). f Helical net diagram showing the positions of ArlB1 (blue dots) and ArlB2 (orange dots) in a two-dimensional plot. Solid black lines show various component helices. Blue and orange lines trace non-helical homopolymeric pseudo-strands of ArlB1 and ArlB2. g Model of the hypothetical assembly of the heteropolymeric archaellum from pre-formed ArlB1-2 dimers that are added to the growing filament in 3-start direction. M, membrane; C, cytosol; red arrow, 3-start direction. For simplicity, the archaellum machinery and the S-layer have not been drawn. Scale bar in d, 10 Å.
