J. Biol. Chem., Vol. 262, Issue 9, 4129-4133, 03, 1987
The binding of myosin heads on heavy meromyosin and assembled myosin to actin in the presence of nucleotides. Measurements by the proteolytic rates method
AM Duong and E Reisler
The initial rates of tryptic digestion at the 50/20-kDa junction in myosin
and myosin subfragment 1 were determined for the free proteins and their
complexes with actin in the presence and absence of MgATP. The proteolytic
reactions were carried out at 24 degrees C and under ionic strength
conditions (mu) adjusted to 35, 60, and 130 mM. The percentages of myosin
heads and myosin subfragment 1 bound to actin in the presence of MgATP were
calculated from the rates of proteolysis for each set of digestion
experiments. In all cases, the myosin heads in the synthetic filaments
showed greater binding to actin than myosin subfragment 1. This binding
difference was most prominent (3-fold) at mu = 130 mM. The binding of heavy
meromyosin (HMM) to actin in the presence of MgADP was measured at 4
degrees C by ultracentrifugation and the proteolytic rates methods.
Ultracentrifugation experiments determined the fraction of HMM molecules
bound to actin in the presence of MgADP, whereas the proteolytic
measurements yielded the information on the fraction of HMM heads bound to
actin. Taken together, these measurements show that a significant fraction
of HMM is bound to actin with only one head in the presence of MgADP under
ionic conditions of 180 and 280 mM.