Cryoatomic force microscopy (cryo-AFM) was used to image phalloidin- stabilized actin filaments adsorbed to mica. The single filaments are clearly shown to be right-handed helical structures with a periodicity of ~38 nm. Even at a moderate concentration (~10 μg/ml), narrow, branched rafts of actin filaments and larger aggregates have been observed. The resolution achieved is sufficient to resolve actin monomers within the filaments. A closer examination of the images shows that the branched rafts are composed of up to three individual filaments with a highly regular lateral registration with a fixed axial shift of ~13 nm. The implications of these higher-order structures are discussed in terms of x-ray fiber diffraction and rheology of actin gels. The cryo-AFM images also indicate that the recently proposed model of left-handed F-actin is likely to be an artifact of preparation and/or low-resolution AFM imaging.
Shao, Z., Shi, D., & Somlyo, A. V. (2000). Cryoatomic force microscopy of filamentous actin. Biophysical Journal, 78(2), 950–958. https://doi.org/10.1016/S0006-3495(00)76652-5