Fluorescent molecular rotor binding to actin

1-(2-Hydroxyethyl)-6-[(2,2-dicyano)vinyl]-2,3,4-trihydroquinoli ne (DCQ) is a fluorescent dye and its fluorescence yield is determined by a freedom of intramolecular rotation. When DCQ was added to an actin solution, its fluorescence intensity increased as the actin molecule transformed from G-actin to F-actin. This fluorescence intensity increase was induced both by the higher binding constant of the dye to F-actin than to G-actin and by the higher fluorescence yield of the dye bound to F-actin than to G-actin. DCQ bound more strongly to Mg(2+)-F-actin than to Ca(2+)-F-actin, resulting in higher fluorescence intensity of DCQ-Mg(2+)-F-actin solution than of DCQ-Ca(2+)-F-actin solution. DCQ did not distinguish Mg(2+)-G-actin and Ca(2+)-G-actin. Thus, DCQ is a good probe for polymerization of actin and polymorphism of F-actin.