STRUCTURAL STUDIES OF MYOSIN-NUCLEOTIDE COMPLEXES - A REVISED MODEL FOR THE MOLECULAR-BASIS OF MUSCLE-CONTRACTION

The structures of the MgADP-beryllium fluoride and MgADP-aluminum fluo ride complexes of the truncated myosin head from Dictyostelium myosin II are reported. These reveal the location of the nucleotide complex a nd define the amino acid residues that form the active site. The terti ary structure of the beryllium fluoride complex is essentially identic al to that seen previously in the three-dimensional structure of chick en skeletal muscle myosin. By contrast, significant domain movements a re observed in the aluminum fluoride complex. These structural finding s form the basis of a revised model for the structural basis of the co ntractile cycle. It is now suggested that the narrow cleft that splits the central 50-kDa segment of the heavy chain provides not only the c ommunication route between the nucleotide-binding pocket and actin but also transmits the conformational change necessary for movement.