Formation of the myosin center dot ADP center dot gallium fluoride complexand its solution structure by small-angle synchrotron X-ray scattering

In the presence of MgADP, a novel phosphate analogue of gallium fluoride (G aFn) forms a ternary complex with the myosin subfragment-1 (S-1), in the sa me way that has been previously reported with aluminum fluoride (AlF4-), be ryllium fluoride (BeFn), scandium fluoride (ScFn), and vanadate (Vi), and t his complex formation may mimic different states along the ATPase kinetic p athway. This novel complex has been characterized and compared with other c omplexes to ascertain whether it forms a transition-state analogue of myosi n ATPase, The complex formed quickly, although several times slower than th e BeFn complex, The half-life of the myosin.ADP.GaFn complex was about 50 h at 4 degrees C. The formation of the myosin ADP GaFn complex was accompani ed by an increase in tryptophane fluorescence, similar to that observed upo n the addition of ATP, but slightly lower than that of the M**.ADP.P-i comp lex. Upon addition of GaFn to acto-myosin ADP, actomyosin did not dissociat e, and the S-1.ADP.GaFn complex was scarcely decomposed by actin, like the AlF4- and ScFn complexes but unlike the BeFn and Vi complexes, The conforma tions at the localized region of SH1, SH2, and RLR, which are very accessib le to the binding of ATP, were studied by fluorescent labeling and chemical modification, and the results suggested that these conformations are very similar to that of the M**.ADP.P-i state. Small-angle X-ray solution scatte ring showed that the radius of gyration value decreases by about 3 Angstrom when S-1 forms an S-1.ADP.GaFn complex, suggesting that the shape of the c omplex becomes compact or rounded in shape, similar to that in the presence of ATP or complexes with other phosphate analogues, and thus mimics the my osin**.ADP.P-i state closely. The overall results may indicate that the com plex mimics a somewhat different transient state from that of other complex es but has a similar global conformation along the ATPase kinetic pathway.