CALCIUM ALONE DOES NOT FULLY ACTIVATE THE THIN FILAMENT FOR S1 BINDING TO RIGOR MYOFIBRILS

Skeletal muscle contraction is regulated by calcium vis, troponin and tropomyosin and appears to involve cooperative activation of cross-bri dge binding to actin. We studied the regulation of fluorescent myosin subfragment 1 (fS1) binding to rigor myofibrils over a wide range of f S1 and calcium levels using highly sensitive imaging techniques, At lo w calcium and low fS1, the fluorescence was restricted to the actin-my osin overlap region. At high calcium and very low fS1, the fluorescenc e was still predominantly in the overlap region. The ratio of nonoverl ap to overlap fluorescence intensity showed that increases in the fS1 level resulted in a shift in maximum fluorescence from the overlap to the nonoverlap region at both low and high calcium; this transition oc curred at lower fS1 levels in myofibrils with high calcium, At a fixed fS1 level, increases in calcium also resulted in a shift in maximum f luorescence from the overlap region to the nonoverlap region. These re sults suggest that calcium alone does not fully activate the thin fila ment for rigor S1 binding and that, even at high calcium, the thin fil ament is not activated along its entire length.