Thermodynamics and Relaxation during Actin Polymerization

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Matthews, Jermey Norman Augustus
Greer, Sandra C
Actin is the most abundant cytoskeletal protein, accounting for 20% of the total protein content in all eukaryotic cells, especially muscle cells. Actin polymerizes from the globular state known as G-actin monomer to the filamentous semi-flexible polymer, F-actin. This polymerization is involved in cell motility, cell signaling, and even in regulation of ion transport. Actin polymerization is known to be regulated in vivo by more than 150 actin-binding proteins, but was shown early on to be capable of thermodynamic regulation and this finding was proven under conditions of varying salt, ATP, initial monomer concentration and temperature. It was shown that actin polymerization is entropically driven, has a "floor" temperature for propagation, and is analogous to a second-order phase transition. The purpose of this work was to measure the effect of pressure on the extent of actin polymerization as a function of temperature. The relaxations of the extent of polymerization after temperature jumps were also measured and analyzed in terms of the initial polymerization rates, rp, as a function of temperature. The effects of "thermal cycling" along the polymerization line were examined. The role of hydrogen bonding was studied by exchanging H2O solvent for D2O, which is known to have stronger "hydrogen" bonding. The results show that actin polymerization under pressure follows the same trend as at atmospheric pressure, exhibiting a floor temperature, Tp, and showing the maximum in the extent of polymerization as first reported by Niranjan et al. New results show that increasing pressure increases Tp and that the volume change of polymerization is positive, a result expected for self-assembly of biological polymers, and varies between 300-500 cm3/mol around 30 0C. The results also revealed the existence of a "ceiling" temperature, Tp2, above which the system re-enters the monomer region of the phase diagram. The relaxation studies revealed that rp correlates with the equilibrium extent of polymerization and shows a peak near the transition temperatures. We also discovered that actin exhibits strong hysteresis upon temperature reversals, especially near the transition temperatures.