Actin plays a variety of roles in eukaryotic cells. To understand the roles of actin and its working mechanisms at the molecular level, it is essential to know the structure of actin. To date, more than 200 actin structures have been elucidated. Using these structures, adoptable conformations of actin are discussed. Actin conformations were classified into four groups: G-form, F-form, C-form and O-form. G-form is a stable conformation, and actin that adopts G-form is rarely converted to C-form or F-form. C-form is unstable and swiftly converted to G-form, and F-form is semi-stable with a barrier between itself and G-form.

Our recent live-cell single-molecule analysis revealed the force-velocity relationship of actin polymerization at the lamellipodium tip. Locally-applied traction force enhanced actin polymerization exclusively at the tip, followed by gradual cell protrusion. The results are consistent with the force-velocity curve predicted by the Brownian ratchet theory. In the lamellipodium, the load on each actin barbed end is kept low by the constant retrograde actin flow and densely-packed filaments. This enables fast reactivity of cell edge protrusion against subtle force changes. Cells appear to use the Brownian ratchet mechanism to harness ‘information’ for their force-directed morphogenesis and migration.

Recurring morphologies observed in various migrating cell types are associated with the directedness and exploratory nature. This article introduces mathematical representation of the morphology dynamics and means by which the simulation results can be systematically compared with microscopy data.

Phosphorylation regulates protein function by altering stereospecific interaction with its substrate or partner proteins. However, recent studies demonstrated that phosphorylation preferably occurs in intrinsically disordered regions (IDRs), which do not have three-dimensional structures. Here, we describe how phosphorylation occurred in IDRs regulates the protein function. Mitotic phosphorylation in the IDRs of Ki-67 and NPM1 promotes or suppresses the liquid-liquid phase separation, respectively, by altering the “charge blockiness” along the polypeptide chain. The phosphorylation-mediated regulation of liquid-liquid phase separation by enhancing or suppressing “charge blockiness”, rather than by modulating stereospecific interaction, provides a new mechanism of protein regulation by post-translational modifications.