Monomers, dimers, and minifilaments of vertebrate skeletal myosin in the presence of sodium pyrophosphate.

The self-assembly of myosin in the presence of sodium pyrophosphate was studied in the pH range between 7.0 and 8.5. As evidenced by sedimentation velocity (S0(20,w) = 6.30 S) and light-scattering measurements (molecular weight of 470 000; radius of gyration = 45 nm), myosin existed in a predominantly monomeric form in the presence of 5 mM sodium pyrophosphate at pH 8.5 and above. The concentration-dependent monomer-dimer equilibrium could be easily shifted toward dimeric species at pH 8.0 in the presence of 5 mM sodium pyrophosphate and 5 mM 2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)-1,3-propanediol. The estimated parameters of the dimeric particles were S0(20,w) between 10 and 11 S, molecular weight of 1.1 X 10(6), and radius of gyration = 52 nm. These results are consistent with a head to tail (parallel) arrangement of staggered myosin molecules in the dimer. At lower pH values (7.5), and in the presence of 10 mM sodium pyrophosphate, the monomer-dimer species were in dynamic equilibrium with myosin minifilaments. At pH 7.0, the minifilaments appeared to be the only detectable species present in solutions of myosin in 5 mM sodium pyrophosphate. The molecular parameters of these minifilaments, including sedimentation and viscosity coefficients, molecular weight, radius of gyration, and morphological appearance, were almost indistinguishable from those obtained for myosin minifilaments prepared in 10 mM citrate-tris(hydroxymethyl)aminomethane at pH 8.0 [Reisler, E., Smith, C., & Seegan, G. (1980) J. Mol. Biol. 143, 129-145]. The equilibrium polymerization reactions of myosin in sodium pyrophosphate are discussed in the context of minifilament assembly.