To obtain proximity information between tropomyosin (Tm) and caldesmon (CaD) on the muscle thin filament, we cloned gizzard αTm and created two single Cys mutants S56C/C190S (56Tm) and D100C/C190S (100Tm). They were labeled with benzophenone maleimide (BPM) and UV-irradiated on thin filaments. One chain of BPM-56Tm and two chains of BPM-100Tm crosslinked to CaD. Only BPM-100Tm crosslinked to actin in the absence and presence of CaD and binding of low ratios of myosin subfragment 1 (S1) prevented the crosslinking. Tm-S1 crosslinks were produced when actin·Tm was saturated with S1. Thus, CaD on the actin·Tm filament is located < 10 A away from Tm amino acids 56 and 100; in the closed state of the actin·Tm filament, Tm residue 100 is located close to the actin surface and is moved further away in the S1-induced open state; in the open state, S1 binds close to Tm.
Golitsina, N. L., & Lehrer, S. S. (1999). Smooth muscle α-tropomyosin crosslinks to caldesmon, to actin and to myosin subfragment 1 on the muscle thin filament. FEBS Letters, 463(1–2), 146–150. https://doi.org/10.1016/S0014-5793(99)01589-6