Cell-cycle-regulated expression of thymosin beta 4 in thymocytes.

Thymosin beta 4 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 beta 4 biosynthesis. The sharp decline of the thymosin beta 4 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 beta 4 concentration, increased again. After 96 h of culture the values returned to those of quiescent cells.