Institute of Biochemistry, Emil-Fischer-Zentrum, Friedrich-Alexander-University, Fahrstr.17, 91054, Erlangen, Germany.
Protein J. 2013 Aug;32(6):484-92. doi: 10.1007/s10930-013-9507-0.
Thymosin β4 is the prototype of β-thymosins and is present in almost every mammalian cell. It is regarded to be the main intracellular G-actin sequestering peptide. Thymosin β4 serves as a specific glutaminyl substrate for guinea pig transglutaminase. In the absence of an appropriate additional aminyl donor an ε-amino group of thymosin β4 serves also as an aminyl substrate and an intramolecular bond is formed concomitantly NH3 (17 Da) is lost. The molecular mass of the product is 4,949.6 Da. This is 16.3 Da less than the molecular mass of thymosin β4 (4,965.9 Da). Digestion with endopeptidases and Edman degradation of the fragments identified the exact position of the ring forming isopeptide bond. In spite of 3 glutaminyl and 9 lysyl residues of thymosin β4 only one isopeptide bond between Lys16 and Gln36 was formed (cyclic thymosin β4). These two amino acid residues are conserved in all β-thymosins. Cyclic thymosin β4 still forms a complex with G-actin albeit the stability of the complex is about one fiftieth of the stability of the thymosin β4 × G-actin complex.
胸腺肽 β4 是 β-胸腺肽的原型，几乎存在于所有哺乳动物细胞中。它被认为是主要的细胞内 G-肌动蛋白隔离肽。胸腺肽 β4 是豚鼠转谷氨酰胺酶的特异性谷氨酰基底物。在没有适当的额外氨基供体的情况下，胸腺肽 β4 的 ε-氨基也可以作为氨基底物，同时形成分子内键，相应地失去 NH3（17 Da）。产物的分子量为 4949.6 Da。这比胸腺肽 β4 的分子量（4965.9 Da）少 16.3 Da。用内切蛋白酶消化和 Edman 降解鉴定片段，确定了形成环异肽键的确切位置。尽管胸腺肽 β4 有 3 个谷氨酰胺残基和 9 个赖氨酸残基，但只有一个赖氨酸 16 和谷氨酰胺 36 之间的异肽键形成（环胸腺肽 β4）。这两个氨基酸残基在所有 β-胸腺肽中都是保守的。尽管环胸腺肽 β4 仍与 G-肌动蛋白形成复合物，但该复合物的稳定性约为胸腺肽 β4×G-肌动蛋白复合物的五十分之一。
