C-terminal variations in beta-thymosin family members specify functional differences in actin-binding properties.

Mammalian cells express several isoforms of beta-thymosin, a major actin monomer sequestering factor, including thymosins beta4, beta10, and beta15. Differences in actin-binding properties of different beta-thymosin family members have not been investigated. We find that thymosin beta15 binds actin with a 2.4-fold higher affinity than does thymosin beta4. Mutational analysis was performed to determine the amino acid differences in thymosin beta15 that specify its increased actin-affinity. Previous work with thymosin beta4 identified an alpha-helical domain, as well as a conserved central motif, as crucial for actin binding. Mutational analysis confirms that these domains are also vital for actin binding in thymosin beta15, but that differences in these domains are not responsible for the variation in actin-binding properties between thymosins beta4 and beta15. Truncation of the unique C-terminal residues in thymosin beta15 inhibits actin binding, suggesting that this domain also has an important role in mediating actin-binding affinity. Replacement of the 10 C-terminal amino acids of thymosin beta15 with those of thymosin beta4 did, however, reduce the actin-binding affinity of the hybrid relative to thymosin beta15. Similarly, replacement of the thymosin beta4 C-terminal amino acids with those of thymosin beta15 led to increased actin binding. We conclude that functional differences between closely related beta-thymosin family members are, in part, specified by the C-terminal variability between these isoforms. Such differences may have consequences for situations where beta-thymosins are differentially expressed as in embryonic development and in cancer. Copyright 2000 Wiley-Liss, Inc.

β-胸腺素家族成员的c端变异决定了肌动蛋白结合特性的功能差异。

哺乳动物细胞表达β-胸腺素的几种亚型，包括胸腺素β4、β10和β15，β-胸腺素是一种主要的肌动蛋白单体螯合因子。尚未研究不同β-胸腺素家族成员的肌动蛋白结合特性的差异。我们发现胸腺素β15结合肌动蛋白的亲和力比胸腺素β4高2.4倍。进行突变分析以确定胸腺素β15中的氨基酸差异，这表明其肌动蛋白亲和力增加。以前关于胸腺素β4的工作确定了α螺旋结构域以及保守的中心基序，它们对于肌动蛋白结合至关重要。突变分析证实，这些结构域对于胸腺素β15中的肌动蛋白结合也是至关重要的，但是这些结构域的差异并不是胸腺素β4和β15之间肌动蛋白结合特性差异的原因。胸腺素β15中独特的C-末端残基的截短抑制了肌动蛋白结合，表明该结构域在介导肌动蛋白结合亲和力中也具有重要作用。然而，用胸腺素β4的氨基酸替换胸腺素β15的10个C-末端氨基酸，相对于胸腺素β15，确实降低了杂交体的肌动蛋白结合亲和力。类似地，用胸腺素β15替换胸腺素β4c末端氨基酸导致肌动蛋白结合增加。我们得出结论，密切相关的β-胸腺素家族成员之间的功能差异，部分是由这些同种型之间的C-末端变异性决定的。这种差异可能对β-胸腺素在胚胎发育和癌症中差异表达的情况产生影响。版权所有2000 Wiley-Liss公司。