Tropomyosin is a coiled-coil protein that polymerizes by head-to-tail interactions in an ionic strength-dependent manner. We produced a recombinant full-length chicken ␣-tropomyosin containing a 5-hydroxytryptophan residue at position 269 (formerly an alanine), 15 residues from the C terminus, and show that its fluorescence intensity specifically reports tropomyosin headto-tail interactions. We used this property to quantitatively study the monomer-polymer equilibrium in tropomyosin and to calculate the equilibrium constant of the head-to-tail interaction as a function of ionic strength. Our results show that the affinity constant changes by almost 2 orders of magnitude over an ionic strength range of 50 mM (between I ؍ 0.045 and 0.095). We were also able to calculate the average polymer length as a function of concentration and ionic strength, which is an important parameter in the interpretation of binding isotherms of tropomyosin with other thin filament proteins such as actin and troponin.Skeletal muscle tropomyosin (Tm) 1 is a 284-residue dimeric in-register coiled-coil protein that plays a central role in the regulation of muscle contraction through its interactions with actin and troponin (Tn) in the thin filament (1-5). Analysis of its primary structure revealed the existence of an extensive and almost perfect heptad repeat (abcdefg) in which hydrophobic residues at positions a and d create a dimerization interface that stabilizes the coiled-coil structure (6, 7). Further analysis of tropomyosin sequences also uncovered seven 39.5-residue pseudo-repeats, which may reflect the tropomyosin 7:1 binding stoichiometry with actin (8). Also associated with both tropomyosin and actin is the troponin complex, which consists of the Ca 2ϩ binding subunit (troponin C), the inhibitory subunit (troponin I), and the tropomyosin binding subunit (TnT). Tn-Tm interactions occur mainly between the C-terminal half of TnT and the region near position 190 of Tm and between the Nterminal half of TnT and the C-terminal of the Tm molecule (for review, see Refs. 9 -13).Muscle tropomyosin high viscosity at low ionic strength was noted from the time of its first isolation (14). Soon thereafter Tsao et al. (15) describe the ionic strength dependence of the viscosity in more detail and Kay and Bailey (16) relate changes in the polymer length with salt concentration and propose that polymerization is due to simple end-to-end or head-to-tail aggregation, which was later confirmed by in-depth studies of cardiac Tm via sedimentation velocity, sedimentation equilibrium, osmometry, viscometry, and optical rotatory dispersion (17).Although muscle Tm is acetylated at its N terminus, recombinant non-fusion Tm (nfTm) expressed in bacteria lacks this modification and does not polymerize or bind to actin (18,19). Recombinant Tm expressed in insect cells (20) or yeast (21) has its N terminus acetylated and is functional. Because the Nacetylated initiation methionine in the native protein occupies an internal position in the coiled-coil struc...