Subcellular Distribution of Thymosin β_4

摘要

The localization of Oregon Green cadaverine-labeled thymosin β_4, its fragments, and variants was investigated in cytoplasm-depleted A431 cells and in microinjected cells without and with fixation. The studied thymosin β_4 variants included substitutions of the lysine residues within the basic cluster (14-KSKLKK-19) and the actin-binding motif (17-LKKTETQ-23). In contrast to Oregon Green cadaverine, none of the variants or fragments of thymosin β_4 could pass the intact nuclear pore of cytoplasm-depleted cells and were hence excluded from the nucleus. However, an equal distribution of all thymosin β_4 variants was observed in living cells. The nuclear localization is neither dependent on the actin-binding ability of thymosin β_4 nor on its basic lysine cluster. The equal distribution of the β-thymosins, the ability of the fragments thymosin β_4~(1-26) and β_4~(27-43) to enter the nucleus in intact cells immediately after injection, and their exclusion from cytoplasm-depleted nuclei make it unlikely that they are transported by a single transport protein. A passive but regulated diffusion could explain the described ability of thymosin β_4 to shuttle into the nucleus.