Dilated cardiomyopathy (DCM), characterized by cardiac dilatation and contractile dysfunction, is a major cause of heart failure. DCM can result from mutations in the gene encoding cardiac alpha-tropomyosin (TM). In order to understand how the dilated cardiomyopathy-causing Glu40Lys mutation in TM affects actomyosin interactions, thin filaments have been reconstituted in muscle ghost fibers by incorporation of labeled Cys707 of myosin subfragment-1 and Cys374 of actin with fluorescent probe 1.5-IAEDANS and alpha-tropomyosin (wild-type or Glu40Lys mutant). For the first time, the effect of these alpha-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. The Glu40Lys mutant TM inhibited these movements at the transition from AM**ADP.Pi to AM state, indicating a decrease of the proportion of the strong-binding sub-states in the actomyosin population. These structural changes are likely to underlie the contractile deficit observed in human dilated cardiomyopathy. (C) 2011 Elsevier Inc. All rights reserved.