ABSTRACT
The effects of calponin on conformational changes in actin caused by modelling of "strong" binding between actin and myosin heads have been studied using polarization fluorimetry. "Strong" binding was modelled by decoration of thin filaments by myosin subfragment I modified by N-ethylmaleimide (NEM-SI) or phosphorylated heavy meromyosin (pHMM). Changes in the actin structure were followed by orientation and mobility of the fluorescent probe--the rhodamine-phalloidin complex. It has been found that calponin cooperatively changes the actin conformation, the maximal conformational changes in actin thin filaments being observed at the calponin/actin molar ratio of about 1:7. The conformational changes in actin induced by NEM-SI and pHMM are typical of strong binding. Calponin inhibited this effect. It is suggested that the mechanism of calponin regulation of smooth muscle contractility is tightly coupled to the inhibition of formation of the stage limiting the rate of ATP hydrolysis by actomyosin.
