Osmotic Properties of Myosin Subfragment 1: Implications of the Mechanism of Muscle Contraction

Abstract

The osmotic behavior of myosin subfragment 1 was studied at 22°C and pH 7.45 in 0.1 m KCl, 2 m m MgCl 2, and 10 m m triethanolamine or in 25 m m phosphate, 2 m m MgCl 2, and 2 m m MgADP. It was found that, in 0.1 m KCl, myosin subfragment 1 behaved as a spheroidal particle, with an average diameter of 8.09 nm, composed of two myosin subfragment 1 molecules. The lower limit of the thermodynamic dimerization constant was estimated to be 3.5 ×10 4 M −1. Above 5 m m as monomer, myosin subfragment 1 departed from the behavior expected of a dimeric spheroidal model because of the onset of a "hydration force." This force measured at the contact distance between particles equals 2.18 × 10 7 dynes/cm 2 and falls off exponentially with a decay distance of 0.27 nm. In 25 m m orthophosphate, myosin subfragment 1, with an increase in the protein osmotic pressure, shifted from the behavior of a sphere to that of a cylinder. Between 1 × 10 5 and 4 × 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 × 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 cylinder with a length to diameter ratio of 2.07 in the absence of MgADP. Under the solution conditions used in this work, S1 never behaved as a fully extended particle.