The polymerization of actin: Extent of polymerization under pressure, volume change of polymerization, and relaxation after temperature jumps

The protein actin can polymerize from monomeric globular G-actin to polymeric filamentary F-actin, under the regulation of thermodynamic variables such as temperature, pressure, and compositions of G-actin and salts. We present here new measurements of the extent of polymerization ( ϕ ) of actin under pressure ( P ) , for rabbit skeletal muscle actin in H 2 O buffer in the presence of adenosine triposphate and calcium ions and at low ( 5 - 15 mM ) KCl concentrations. We measured ϕ using pyrene-labeled actin, as a function of time ( t ) and temperature ( T ) , for samples of fixed concentrations of initial G-actin and KCl and at fixed pressure. The ϕ ( T , P ) measurements at equilibrium have the same form as reported previously at 1 atm : low levels of polymerization at low temperatures, representing dimerization of the actin; an increase in ϕ at the polymerization temperature ( T p ) ; a maximum in ϕ ( T ) above T p with a decrease in ϕ ( T ) beyond the maximum, indicating a depolymerization at higher T . From ϕ ( T , P ) at temperatures below T p , we estimate the change in volume for the dimerization of actin, Δ V dim , to be − 307 ± 10 ml ∕ mol at 279 K . The change of T p with pressure d T p ∕ d P = ( 0.3015 ± 0.0009 ) K ∕ MPa = ( 30.15 ± 0.09 ) mK ∕ atm . The ϕ ( T , P ) data at higher T indicate the change in volume on propagation, Δ V prop , to be + 401 ± 48 ml ∕ mol at 301 K . The ϕ ( t ) measurements yield initial relaxation times r p ( T ) that reflect the behavior of ϕ ( T ) and support the presence of a depolymerization temperature. We also measured the density of polymerizing actin with a vibrating tube density meter, the results of which confirm that the data from this instrument are affected by viscosity changes and can be erroneous.