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Actin-bound nucleotide/divalent cation interactions.

Abstract：

:At this point, it may be worthwhile to list, in summary form, the important aspects of divalent cation and nucleotide binding to actin that have been reviewed here: 1) High affinity divalent cation binding to actin is very tight, with equilibrium dissociation constant KCa approximately 1 nM and KMg approximately 5 nM at pH 7.0. 2) The binding kinetics of Ca++ are diffusion limited. Dissociation is slow, with k-Ca approximately 0.015 sec at pH 7.0 (and low ionic strength). 3) The binding kinetics of Mg++ are limited by the characteristics of the Mg++ aquo-ion and are much slower than for Ca++; k-Mg approximately 0.0012 at pH 7.0. 4) Increase in pH or ionic strength weakens divalent cation binding at the high affinity site, primarily by increasing k-Ca and k-Mg. 5) Exchange of Mg++ for Ca++ (or vice versa) at the high affinity site is by a competitive pseudo-first order process with an apparent rate constant (kapp) intermediate between k-Ca and k-Mg and dependent upon the cation concentration ratio [Ca]/[Mg] present. 6) High affinity ATP binding is modulated by the high affinity divalent cation. The cation concentration range over which this modulation occurs is about 100-fold higher for Mg++ than for Ca++, again because of the different characteristics of the Mg++ and Ca++ aquo-ions. 7) At low divalent cation concentrations, ATP dissociation from actin is limited by dissociation of the tightly-bound divalent cation. 8) At high divalent cation concentrations, ATP dissociation probably occurs via dissociation of the divalent cation-nucleotide complex and is quite slow, with dissociation rate constant approximately 0.0005 sec-1. 9) Competitive nucleotide exchange on actin may be described by a pseudo-first order model analogous to that for divalent cation exchange. The pseudo-first order rate constants depend upon the divalent cation concentration. The overall nucleotide exchange rate constant kex depends upon these constants and the solution nucleotide concentration ratio, e.g. [ATP]/[ADP]. The following circumstances develop from the characteristics of the high affinity binding of divalent cation and nucleotide to actin: 1) The standard methods for actin preparation convert in vivo Mg-actin into Ca-actin. 2) Converting Ca-actin back to Mg-actin is not easy. A very low ratio of [Ca]/[Mg] is necessary, which usually requires the use of Ca-cheltors, and a long time (5-10 min) must be allowed for complete exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

journal_name

Adv Exp Med Biol

journal_title

Advances in experimental medicine and biology

authors

Gershman LC,Selden LA,Kinosian HJ,Estes JE

doi

10.1007/978-1-4615-2578-3_4

subject

Has Abstract

pub_date

1994-01-01 00:00:00

pages

35-49

eissn

0065-2598

issn

2214-8019

journal_volume

358

pub_type

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Actin-bound nucleotide/divalent cation interactions.