Mf. Carlier et al., T-BETA(4) IS NOT A SIMPLE G-ACTIN SEQUESTERING PROTEIN AND INTERACTS WITH F-ACTIN AT HIGH-CONCENTRATION, The Journal of biological chemistry, 271(16), 1996, pp. 9231-9239
Thymosin beta(4) is acknowledged as a major G-actin binding protein ma
intaining a pool of unassembled actin in motile vertebrate cells. We h
ave examined the function of T beta(4) in actin assembly in the high r
ange of concentrations (up to 300 mu M) at which T beta(4) is found ir
e highly motile blood cells. T beta(4) behaves as a simple G-actin seq
uestering protein only in a range of low concentrations (<20 mu M). As
the concentration of T beta(4) increases, its ability to depolymerize
F-actin decreases, due to its interaction with F-actin. The T beta(4)
-actin can be incorporated, in low molar ratios, into F-actin, and can
be cross-linked in F-actin using 1-ethyl-3-(3-dimethylaminopropyl)car
bodiimide. As a result of the copolymerization of actin and T beta(4)-
actin complex, the critical concentration is the sum of free G-actin a
nd T beta(4)-G-actin concentrations at steady state, and the partial c
ritical concentration of G-actin is decreased by T beta(4)-G-actin com
plex. The incorporation of T beta(4)-actin in F-actin is associated to
a structural change of the filaments and eventually leads to their tw
isting around each other. In conclusion, T beta(4) is not a simple pas
sive actin-sequestering agent, and at high concentrations the ability
of T beta(4)-actin to copolymerize with actin reduces the sequestering
activity of G-actin-binding proteins. These results question the eval
uation of the unassembled actin in motile cells. They account for obse
rvations made on living fibroblasts overexpressing beta-thymosins.