THE MYOSIN MOLECULE - CHARGE RESPONSE TO NUCLEOTIDE-BINDING

A decrease in the net fixed electric charge in the A-bands of cross-st riated muscle was observed by Bartels and Elliott [2,10] when the musc le went from the rigor to the relaxed condition. The current work loca lises the source of the charge decrease by following the net charge on myosin (in the form of concentrated gels) and also myosin rod and lig ht meromyosin gels when the gels are exposed to different concentratio ns of ATP. The work includes a study of muscle A-bands when the muscle is exposed to the same variations in ATP concentrations as the protei n gels. The work shows that (i) Only 100-200 mum ATP is needed to init iate the charge decrease between the rigor and relaxed conditions; (ii ) the effect of ATP is seen in the muscle A-band and the myosin and my osin rod gels, but not in LMM gels; (iii) pyrophosphate (PPi) shows a similar charge effect to ATP. ADP does not affect the charge on myosin gels, on the other hand. The results suggest that the charge decrease caused by ATP or PPi is due to ligand interaction with one or more si tes on the myosin molecule. This interaction causes a disseminated eff ect in the protein, and a consequent loss in net negative charge eithe r by a decrease in the absorption, of anions to Saroff sites on the pr otein, or, less probably, by an increase in the absorption of cations at those sites.