High-resolution AFM of AmtB. (A) Height image of the upper layer of a double-layered 2D crystal. An AmtB trimer is marked by the broken circle. The area indicated by the broken frame is magnified in (B). (C) Correlation average of 213 AmtB trimers and (D) with three-fold symmetrization. The arrows mark the prominent protrusions near the three-fold axis of the AmtB trimer and the arrowheads indicate the smaller, peripheral protrusions; the dashed lines indicate apparent pseudo-two-fold symmetry axes. The topographs in (A,B) are shown in relief, tilted by 15°. Scale bar in (A): 15 nm. Frame size in (B): 12.3 nm. Vertical brightness ranges: 1.3 nm (A,B) and 1.2 nm (C,D). Frame sizes in (C,D): 11.2 nm.