Abstract
Deoxyribonuclease I (DNase I) forms a 1:1 complex with globular actin (G-actin) and also will depolymerize filamentous actin (F-actin) to form a 1:1 complex. The effect of DNase I on the exchange of the actin nucleotide has been investigated. When DNase I is added to G-actin, the rate of nucleotide exchange is decreased from 1.16 +/- 0.25 X 10(-4) s-1 to 0.28 +/- 0.09 X 10(-4) s-1 (0 degrees C). The presence of ATP or ADP in the actin has little effect on the rate of exchange of the nucleotide for ATP. This suggests that the weaker affinity of ADP than ATP for actin is due to a slower association rate of ADP. The rate of the nucleotide exchange in the actinDNase I complex is increased by the addition of NaCl or MgCl2. When DNase I is added to F-actin, the rate of nucleotide exchange (6.2 +/- 1.6 X 10(-4) x-1, 0 degrees C) is similar to the rate of depolymerization as measured by loss of viscosity. The actinDNase I complex formed by depolymerization of F-actin exchanges nucleotide at a 4-fold faster rate than the G-actinDNase I complex in the same ionic conditions. This and other experiments suggest that DNase I binds first to F-actin before dissociating the monomer from the filament. These results are discussed in terms of possible mechanisms of action depolymerization.
Highlights
Deoxyribonuclease I (DNase I) forms a 1:1 complex with globularactin (G-actin)and will depolymerize filamentous actin (F-actin) to form a 1:1 complex
WhenDNase I bindstoG-actin,therate of nucleotide exchange is reduced by a factor of about 4
The observation that the actin.DNase I complex formed by depolym
Summary
When thetwo types of actin arecomplexed with DNase I, the free nucleotide is included during the depolymerization, the rates and extentof exchange are similar both in the presence rate of depolymerization is the same (as measuredby loss of of NaCl and MgC12,as seenin Table 11, and in the absenceof viscosity; Hitchcock et al, 1976) and the ADP is retained in the NaCl and MgC1, (results not shown). DNaseI complex prepared by depolymerthat the rateof nucleotide exchange of the complex made by ization of F-actin was used without further purification since depolymerization of F-actin is about four times as fasat s that it had been shown previously that when it was applied to a made by complexing DNase I with G-ADP or G-ATP-actin. Binding of DNase I to F-Actin-The experiments described above showthat DNaseI can form twotypes of complex with actin monomer having different rates of nucleotide exchange depending on whether the actinis initially in the fdamentous.
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