Cell‐cycle‐regulated expression of thymosin β4 in thymocytes

Thymosin β4 belongs to a family of ubiquitous peptides present at a high cellular content but still with an unknown intracellular function. The expression of this peptide was studied in concanavalin‐A‐stimulated, proliferating rat thymocytes during cell cycle progression. An early, transient 10‐fold increase of the peptide occurred 1 h after stimulation without elevation of the corresponding mRNA level. This increase coincided with that of thymosin β4 biosynthesis. The sharp decline of the thymosin β4 content was not due to a secretion of the peptide into the medium. During S phase and mitosis, the biosynthetic rates as well as mRNA content, but not the cellular thymosin β4 concentration, increased again. After 96 h of culture the values returned to those of quiescent cells.