Mj. Redowicz et al., NUCLEOTIDES INCREASE THE INTERNAL FLEXIBILITY OF FILAMENTS OF DEPHOSPHORYLATED ACANTHAMOEBA MYOSIN-II, The Journal of biological chemistry, 271(21), 1996, pp. 12401-12407
The actin-activated Mg2+-ATPase activity of Acanthamoeba myosin II min
ifilaments is dependent both on Mg2+ concentration and on the state of
phosphorylation of three serine sites at the C-terminal end of the he
avy chains. Previous electric birefringence experiments on minifilamen
ts showed a large dependence of signal amplitude on the phosphorylatio
n state and Mg2+ concentration, consistent with large changes in filam
ent flexibility. These observations suggested that minifilament stiffn
ess was important for function. We now report that the binding of nucl
eotides to dephosphorylated minifilaments at Mg2+ concentrations neede
d for optimal activity increases the flexibility by about 10-fold, as
inferred from the birefringence signal amplitude increase. An increase
in flexibility with nucleotide binding is not observed for dephosphor
ylated minifilaments at lower Mg2+ concentrations or for phosphorylate
d minifilaments at any Mg2+ concentration examined. The relaxation tim
es for minifilament rotations that are sensitive to the conformation m
yosin heads are also observed to depend on phosphorylation, Mg2+ conce
ntration, and nucleotide binding. These latter experiments indicate th
at the actin activated Mg2+-ATPase activity of Acanthamoeba myosin II
correlates with both changes in myosin head conformation and the abili
ty of minifilaments to cycle between stiff and flexible conformations
coupled to nucleotide binding and release.