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The effects resulting from the removal of the N-terminus of heavy meromyosin (HMM) A1 light chain by papain digestion are investigated. The fluorometry of TRITCphalloidin labelled actin in ghost fibers is used as a tool for sensing conformational changes of rigor complex of phosphorylated and dephosphorylated HMM with actin filament. The experiments were performed both under conditions assuring saturation of RLC with magnesium cation (4 mM EGTA) or calcium cation (0.1 mM CaCI2), and in constant presence of 1 mM magnesium chloride. HMM native and with A1 shortened from the N-terminus is used. As it was observed previously rigor complex of actin filament and native HMM shows sensitivity to the kind of cation saturating RLC and to the phosphorylation status of RLC. In particular, the sin20 parameter of actin bound rhodamine-phalloidin fluorescence polarization representing roughly the flexibility of actin filament HMM complex changes significantly with the changes of RLC phosphorylation and cation saturation. Removal of the N-terminus of A1 reduces this sensitivity to cation and phosphorylation both in the case of dephosphorylated and phosphorylated HMM. Our results suggest that the N-terminus of A1 plays significant role in the rigor interaction of myosin heads with actin and is involved in modulatory function of RLC in this interaction.