The shortening of the N-terminus of myosin essential light chain A1 influences the interaction of heavy meromyosin with actin

The effects resulting from the removal of the N-terminus of heavy meromyosi n (HMM) Al light chain by papain digestion are investigated. The fluorometr y of TRITC-phalloidin labelled actin in ghost fibers is used as a tool for sensing conformational changes of rigor complex of phosphorylated and depho sphorylated HMM with actin filament. The experiments were performed both un der conditions assuring saturation of RLC with magnesium cation (4 mM EGTA) or calcium cation (0.1 mM CaCl2), and in constant presence of 1 mM magnesi um chloride. HMM native and with Al shortened from the N-terminus is used. As it was observed previously rigor complex of actin filament and native HM M shows sensitivity to the kind of cation saturating RLC and to the phospho rylation status of RLC. In particular, the sin(2)theta parameter of actin b ound rhodamine-phalloidin fluorescence polarization representing roughly th e flexibility of actin filament HMM complex changes significantly with the changes of RLC phosphorylation and cation saturation. Removal of the N-term inus of Al reduces this sensitivity to cation and phosphorylation both in t he case of dephosphorylated and phosphorylated HMM. Our results suggest tha t the N-terminus of Al plays significant role in the rigor interaction of m yosin heads with actin and is involved in modulatory function of RLC in thi s interaction.