Cooperative and non-cooperative conformational changes of F-actin induced by cofilin

• Mobility of MTSL attached to C374 in F-actin became high upon addition of cofilin.
• Change of motility of MTSL attached to C374 with cofilin-binding was cooperative.
• Mobility of MTSL attached to V43C in F-actin became high upon addition of cofilin.
• Change of motility of MTSL attached to V43C with cofilin-binding was linear.

Cofilin is an actin-binding protein that promotes F-actin depolymerization. It is well-known that cofilin-coated F-actin is more twisted than naked F-actin, and that the protomer is more tilted. However, the means by which the local changes induced by the binding of individual cofilin proteins proceed to the global conformational changes of the whole F-actin molecule remain unknown. Here we investigated the cofilin-induced changes in several parts of F-actin, through site-directed spin-label electron paramagnetic resonance spectroscopy analyses of recombinant actins containing single reactive cysteines. We found that the global, cooperative conformational changes induced by cofilin-binding, which were detected by the spin-label attached to the Cys374 residue, occurred without the detachment of the D-loop in subdomain 2 from the neighboring protomer. The two processes of local and global changes do not necessarily proceed in sequence.