4 is not a simple G-actin sequestering protein and interacts with F-actin at high concentration

Marie France Carlier, Dominique Didry, Inge Erk, Jean Lepault, Marleen L. Van Troys, Joël Vandekerckhove, Irina Perelroizen, Helen Yin, Yukio Doi, Dominique Pantaloni

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71 Scopus citations


Thymosin β4 is acknowledged as a major G-actin binding protein maintaining a pool of unassembled actin in motile vertebrate cells. We have examined the function of Tβ4 in actin assembly in the high range of concentrations (up to 300 μM) at which Tβ4 is found in highly motile blood cells. Tβ4 behaves as a simple G-actin sequestering protein only in a range of low concentrations (<20 μM). As the concentration of Tβ4 increases, its ability to depolymerize F-actin decreases, due to its interaction with F-actin. The Tβ4-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)carbo-diimide. As a result of the copolymerization of actin and Tβ4-actin complex, the critical concentration is the sum of free G-actin and Tβ4-G-actin concentrations at steady state, and the partial critical concentration of G-actin is decreased by Tβ4-G-actin complex. The incorporation of Tβ4-actin in F-actin is associated to a structural change of the filaments and eventually leads to their twisting around each other. In conclusion, Tβ4 is not a simple passive actin-sequestering agent, and at high concentrations the ability of Tβ4-actin to copolymerize with actin reduces the sequestering activity of G-actin-binding proteins. These results question the evaluation of the unassembled actin in motile cells. They account for observations made on living fibroblasts overexpressing β-thymosins.

Original languageEnglish (US)
Pages (from-to)9231-9239
Number of pages9
JournalJournal of Biological Chemistry
Issue number16
StatePublished - Apr 19 1996

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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