Interaction of actin with dansyl-tropomyosin

5-[Dimethylamino]naphthalene-1-sulfonyl chloride (dansyl chloride) reacts with rabbit skeletal muscle tropomyosin (TM) to yield a highly fluorescent product, DNS-TM. The extent of modification of TM can be regulated over a wide range, 0.3-15.5 dansyl groups per TM, depending upon the temperature and duration of the reaction. However, under all conditions employed, about 15 different fluorescent tryptic peptides of TM were produced. DNS-TM undergoes end-to-end polymerization at low ionic strengths, but to a somewhat lesser extent than unlabelled TM does. DNS-TM also binds muscle F-actin. This interaction may be followed fluorimetrically by observing a blue-shift in emission maximum, an increase in fluorescence intensity or an increase in fluorescence polarization of the DNS-TM complex with F-actin.