OSMOTIC PROPERTIES OF MYOSIN SUBFRAGMENT .1. IMPLICATIONS OF THE MECHANISM OF MUSCLE-CONTRACTION

The osmotic behavior of myosin subfragment 1 was studied at 22 degrees C and pH 7.45 in 0.1 m KCl, 2 mm MgCl2, and 10 m;rn triethanolamine o r in 25 mm phosphate, 2 mm MgCl2, and 2 mm MgADP. It was found that, i n 0.1 m KCl, myosin subfragment 1 behaved as a spheroidal particle, wi th an average diameter of 8.09 nm, composed of two myosin subfragment 1 molecules. The lower limit of the thermodynamic dimerization constan t was estimated to be 3.5 x 10(4) M(-1). Above 5 mm as monomer, myosin subfragment 1 departed from the behavior expected of a dimeric sphero idal model because of the onset of a ''hydration force.'' This force m easured at the contact distance between particles equals 2.18 x 10(7) dynes/cm(2) and falls off exponentially with a decay distance of 0.27 nn. In 25 mm orthophosphate, myosin subfragment 1, with an increase in the protein osmotic pressure, shifted from the behavior of a sphere t o that of a cylinder. Between 1 x 10(5) and 4 x 10(5) dynes/cm(2), the behavior of myosin subfragment 1 was different in the presence and in the absence of MgADP. In particular, at 1.8 x 10(5) dynes/cm(2), the protein osmotic pressure in frog muscle, myosin subfragment 1 behaved as a sphere of 3.21-nm radius in the presence of MgADP and as a cylind er with a length to diameter ratio of 2.07 in the absence of MgADP. Un der the solution conditions used in this work, S1 never behaved as a f ully extended particle. (C) 1995 Academic Press, Inc.