Quantitative analysis of tropomyosin linear polymerization equilibrium as a function of ionic strength.

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 alpha-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 head-to-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.