Orientation and mobility of acrylodan fluorescent probe specifically bound to caldesmon Cys580 incorporated into muscle ghost fibers decorated with myosin S1 and containing tropomysoin was studied in the presence or absence of MgADP, MgAMP-PNP, MgATP gamma S or MgATP. Modeling of various intermediate states of actomyosin has shown discrete changes in orientation and mobility of the dye dipoles which is the evidence for multistep changes in the structural changes of caldesmon during the ATPase hydrolysis cycle. It is suggested that SI interaction with actin results in nucleotide-dependent displacement of the C-terminal part of caldesmon molecule and changes in its mobility. Thus inhibition of the actomyosin ATPase activity may be due to changes in caldesmon position on the thin filament and its interaction with actin. Our new findings described in the present paper as well as those published recently elsewhere might conciliate the two existing models of molecular mechanism of inhibition of the actomyosin ATPase by caldesmon. (c) 2007 Elsevier Inc. All rights reserved.