Über Actin-Nucleotide und die Funktion und Bindung der Nucleotidphosphate im G- und F-Actin

Abstract

1. If pure F-actin-ADP * which is free of enzymes is depolymerized G-actin-ATP * arises in the presence of 10-4 M ATP, G-actin-ITP * in the presence of 10-4 M ITP, and G-actin-ADP in the presence of 10-4 M ADP. If the depolymerization takes place in the absence of free nucleotide phosphate G-actin-ADP also arises. If G-actin-ADP is added to a solution containing 10-4 M ITP or ATP the bound ADP is exchanged with ATP respectively with ITP (Section II). 2. G-actin-ATP and G-actin-ITP polymerize to F-actin-ADP and to F-actin-IDP respectively by splitting off the γ-phosphate of the ATP or ITP. G-actin-ADP polymerizes to F-actin-ADP without splitting off phosphate. The polymerization of G-actin-ADP is as complete as the polymerization of G-actin-ATP; but the process is perceptibly shower (Section II). 3. G-actin that is not bound to a nucleotide phosphate does not polymerize (Section II). 4. G-actin-ADP in the absence of free ADP spontaneously disintegrates in a half-life of 70 minutes to yield G-actin and ADP. If the dissociating ADP is continuously removed by being bound to Dowex 1 × 10 the half life drops to 7 to 8 minutes. In the presence of Dowex G-actin-ATP disintegrates in a half life of 240 minutes (Section III). 5. The disintegration of G-actin-ADP takes place in two stages. A reversible dissociation into ADP and G-actin I is followed by an irreversible denaturation of G-actin I to G-actin II in a half life ~ 12 minutes. Contrary to actin I G-actin II even on the addition of ATP no longer polymerizes. The difference in the half life of pure G-actin-ADP on the hand and of G-actin-ADP+ADP as well as G-actin-ATP on the other must be attributed to the relatively high equilibrium concentration of G-actin I in the first case and of the relatively slight equilibrium concentration of G-actin I in the second case (Section IV). 6. If the alkaline earth of G-actin is blocked by 10-3 M EDTA G-actin-ATP disintegrates in a half life ∼ 3 minutes and G-actin-ADP in a half life ∼ 0,3 minutes. On the other hand, the stability of F-actin-ADP is not noticeably affected (Section V). 7. Through a two hour rapid dialysis in the presence of 10-4 M ATP the KCl-content of an F-actin-ADP solution drops to 5 × 10-4 M KCl. In spite of this the depolymerization and exchange of ADP with ATP is finished not before 40 hours if the solution remains at rest. If, however, the actin solution containing 5 × 10-4 M KCl is treated with the Teflon homogenizer for about 30 sec. depolymerization and ADP-ATP-exchange occur immediately. On the contrary, F-actin-ADP in 10–1 M KCl solution is not affected at all by a treatment with the Teflon-homogenizer. Apparently the decrease of the KClconcentration from 10-1 M to 5 × 10-4 M considerably diminishes the strength of the bond between the actin monomers without immediately destroying the F-actin arrangement. The immediate ADP-ATP-exchange after the mechanical destruction of the F-actin arrangement proves that this exchange in F-actin does not take place only because of steric hindrance. ADP is present in F-actin apparently between the individual monomers so that EDTA, ATP and enzymes affecting ATP cannot approach ADP. Consequently it is not necessary to assume that the extraordinary stability of F-actin-ADP is due to a special kind of bond between actin monomers and nucleotide phosphate (Section V). 8. In the appendix it is shown that G-actin-ADP does not polymerize 15′ after preparation if the aceton dried muscle powder is prepared at pH 8 to 9 instead of pH ∼ 7.