Perelroizen I, Carlier M F, Pantaloni D
Laboratoire d'Enzymologie du C.N.R.S., Gif-sur-Yvette, France.
J Biol Chem. 1995 Jan 27;270(4):1501-8. doi: 10.1074/jbc.270.4.1501.
The effect of profilin, a G-actin binding protein, on the mechanism of exchange of the tightly bound metal ion and nucleotide on G-actin, has been investigated. 1) In low ionic strength buffer, profilin increases the rates of Ca2+ and Mg2+ dissociation from G-actin 250- and 50-fold, respectively. On the profilin-actin complex as well as on G-actin alone, nucleotide exchange is dependent on the concentration of divalent metal ion and is kinetically limited, at low concentration of metal ion, by the dissociation of the metal ion. 2) Under physiological ionic conditions, nucleotide exchange on G-actin is 1 order of magnitude faster than at low ionic strength. The rate of MgATP dissociation is increased by profilin from 0.05 s-1 to 2 s-1, the rate of MgADP dissociation is increased from 0.2 s-1 to 24 s-1. The dependences of the exchange rates on profilin concentration are consistent with a high affinity (5 x 10(6) to 10(7) M-1) of profilin for ATP-G-actin, and a 20-fold lower affinity for ADP-G-actin. Profilin binding to actin lowers the affinity of metal-nucleotide by about 1 order of magnitude. These results restrain the possible roles of profilin in actin assembly in vivo.
已对一种G-肌动蛋白结合蛋白——肌动蛋白单体结合蛋白对G-肌动蛋白上紧密结合的金属离子和核苷酸交换机制的影响进行了研究。1）在低离子强度缓冲液中，肌动蛋白单体结合蛋白分别使Ca2+和Mg2+从G-肌动蛋白上解离的速率提高了250倍和50倍。在肌动蛋白单体结合蛋白-肌动蛋白复合物以及单独的G-肌动蛋白上，核苷酸交换均取决于二价金属离子的浓度，并且在低浓度金属离子时，受金属离子解离的动力学限制。2）在生理离子条件下，G-肌动蛋白上的核苷酸交换比在低离子强度时快1个数量级。肌动蛋白单体结合蛋白使MgATP解离速率从0.05 s-1提高到2 s-1，使MgADP解离速率从0.2 s-1提高到24 s-1。交换速率对肌动蛋白单体结合蛋白浓度的依赖性与肌动蛋白单体结合蛋白对ATP-G-肌动蛋白的高亲和力（5×10(6)至10(7) M-1）以及对ADP-G-肌动蛋白低20倍的亲和力一致。肌动蛋白单体结合蛋白与肌动蛋白的结合使金属-核苷酸的亲和力降低约1个数量级。这些结果限制了肌动蛋白单体结合蛋白在体内肌动蛋白组装中可能发挥的作用。
