Fluorescence from pyrene-labeled native and reconstituted chicken gizzard tropomyosins

Sulfhydryl groups at Cys-36 on the β chain and at Cys-190 on the γ chain of chicken gizzard tropomyosin were reacted with the pyrene-containing sulfhydryl-specific reagents N-(1-pyrenyl)iodoacetamide and N-(1-pyrenyl)maleimide. Tropomyosin prepared and labeled under nondenaturing conditions displayed significant pyrene monomer emission but low levels of pyrene excimer fluorescence. In contrast, tropomyosin subjected to denaturation and renaturation prior to labeling, or labeled in the denatured state prior to renaturation, displayed considerable excimer emission. Furthermore, labeling of isolated β or γ chains in denaturant, followed by reconstitution, gave separate samples of ββ- and γγ-tropomyosin that exhibited even greater pyrene excimer to monomer emission ratios. As pyrene excimers can form only when an excited pyrene is immediately adjacent to a ground state pyrene, i.e., when the labeled Cys residues on the two chains in a tropomyosin coiled coil share the same cross section, these results support conclusions based upon chemical crosslinking studies [ C. Sanders, L. D. Burtnick, and L. B. Smillie (1986) J. Biol. Chem. 261, 12774–12778 ] that native gizzard tropomyosin exists predominantly as a βγ-heterodimer. In addition, the low degree of labeling of native gizzard tropomyosin and the differences in degrees of labeling of ββ- and γγ-tropomyosins in the absence of denaturants reflect on the accessibilities of the sulfhydryl groups in these tropomyosin isoforms. Circular dichroism measurements indicate that the labeled proteins form stable coiled coil structures that have thermal stabilites comparable to that of the native protein.