Kinetics of self-assembly of alpha alpha-tropomyosin coiled coils from unfolded chains

The two-chain coiled-coil structural motif is found in fibrous muscle proteins and leucine zippers. Unfolding/refolding studies abound, but none establishes the time scale or mechanism of structural assembly from separated, unfolded chains. Stopped-flow circular dichroism studies of such refolding of alpha-tropomyosin chains are reported here. The backbone spectral region (222 nm) reveals a fast phase (less than 0.04 s), yielding an intermediate possessing approximately 70% of the equilibrium helix content. A subsequent slow phase is first order [k-1 (20 degrees C) = 1.67 s, Ea = 12.7 kcal.mol-1 (1 kcal = 4.18 kJ)], so dimerization is fast. The same rate constant characterizes folding in the Cys-190 crosslinked chains, so the intermediate has parallel and nearly registered chains. The tyrosine spectral region (280 nm) reveals only a fast phase, so these six chain sites are native in the intermediate.