Smooth muscle α‐tropomyosin crosslinks to caldesmon, to actin and to myosin subfragment 1 on the muscle thin filament

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Abstract
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 Å 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.