title: 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 degrees C and pH 7.45 in 0.1 m KCl, 2 mm MgCl2, and 10 mm triethanolamine or in 25 mm phosphate, 2 mm MgCl2, and 2 mm 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 x 10(4) M-1. Above 5 mm 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 x 10(7) dynes/cm2 and falls off exponentially with a decay distance of 0.27 nm. In 25 mm 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 x 10(5) and 4 x 10(5) dynes/cm2, 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/cm2, 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.
authors.affiliation: Dipartimento di Biochimica e Biologia Molecolare, Università di Ferrara, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Grazi
authors.forename: E
authors.initials: E
authors.lastname: Magri
authors.forename: C
authors.initials: C
authors.lastname: Schwienbacher
authors.forename: G
authors.initials: G
authors.lastname: Trombetta
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D001001:Anura; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000502:physiology*; D009132:Muscles / Q000737:chemistry / Q000502:physiology; D015879:Myosin Subfragments / Q000737:chemistry* / Q000502:physiology*; D009995:Osmosis; D009997:Osmotic Pressure; D010710:Phosphates; D011189:Potassium Chloride; D011487:Protein Conformation; D012996:Solutions; D013816:Thermodynamics
chemical_list: D015879:Myosin Subfragments; D010710:Phosphates; D012996:Solutions; D000244:Adenosine Diphosphate; D011189:Potassium Chloride