Evaluating atomic models of F-actin with an undecagold-tagged phalloidin derivative.

We have prepared an undecagold-tagged phalloidin derivative to determine this mushroom toxin's binding site and orientation within the F-actin filament by scanning transmission electron microscopy (STEM) and 3-D helical reconstruction. Remarkably, when stoichiometrically bound to F-actin, the undecagold moiety of the derivative could be directly visualized by STEM along the two half-staggered long-pitch helical strands of single filaments. Most importantly, the structural data obtained when combined with various biochemical constraints enabled us to critically evaluate two distinct atomic models of the F-actin filament (i.e. the Holmes-Lorenz versus the Schutt-Lindberg model). Taken together, our data are in excellent agreement with the Holmes-Lorenz model.