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Figure 4.
Figure 4. TcpA Crystal Lattice and Structure-Based Model of
TCP Filament(A) Crystal lattice showing TcpA subunits arranged
in hexagonally packed fibers. The three molecules in the
asymmetric unit are colored red and appear at different
levels.(B) A single crystallographic fiber showing that the
N-terminal α helices face the center of the fiber and have the
same polarity. The three strands of the helical fiber are
colored red, blue, and yellow. Two subunits are shown for each
start, with the second one rotated 60° counterclockwise
relative to the first and translated 17.85 Å along the
fiber axis (i.e., into the page).(C) Side view of a single
crystallographic fiber showing the left-handed three-start
helix, fiber dimensions, and helical symmetry. Six subunits are
shown in one complete turn for each helical strand of the
three-start helix assembly.(D) Hydrophobic interface connecting
subunits within each strand of the left-handed three-start
helices. Side chains on α2 of one subunit are shown as yellow
ball-and-stick representations on an orange ribbon, and side
chains on α3 and α4 are shown in cyan on a green ribbon.
Relevant oxygen atoms are colored red, and nitrogens are blue.
In addition to the hydrophobic interactions, two hydrogen bonds
link the subunits (Tyr51:OH to Leu176:O, and Thr125:OH to
Leu76:O) as indicated by white spheres. The disulfide-bound
cysteines are colored pink with yellow sulfur atoms.(E and F)
Top view (E) and side view (F) of the structure-based TCP model
derived from the symmetry determined by EM analysis and the
packing arrangement seen in the crystallographic fibers. An
extended α-helical tail has been added to the N terminus using
the coordinates of the PAK pilin α1-N. The dimensions are shown
for comparison with the crystallographic fiber in (C).
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