EFFECTS OF THYMOSIN BETA-4 AND THYMOSIN BETA-10 ON ACTIN STRUCTURES IN LIVING CELLS

The beta-thymosins are a family of small proteins originally isolated from the thymus. Recently, two of the major mammalian isoforms, thymos in beta 4 (T beta 4) and thymosin beta 10 (T beta 10), are identified as significant actin monomer sequestering proteins which may be involv ed in regulating actin filament assembly. To study the cellular functi on of beta-thymosins, we have used isoform-specific antibodies to dete rmine their concentration and intracellular distribution, and examined the effects of inducing overexpression of T beta 4 and T beta 10 on a ctin filament structures. Immunofluorescence labeling of peritoneal ma crophages showed that both beta-thymosins are uniformly distributed wi thin the cytoplasm. cDNA-mediated overexpression of beta-thymosins in CV1 fibroblasts induced extensive loss of phalloidin-stained actin str ess fibers. Stress fibers in the cell center were more susceptible tha n those at the periphery. There was a decrease in the number of focal adhesions, as evidenced by a decrease in discrete vinculin staining an d an increase in diffuse vinculin fluorescence. The majority of the tr ansfected cells had normal shape in spite of extensive loss of actin f ilaments. Occasionally, cells overexpressing beta-thymosin were observ ed to divide. In these cells, beta-thymosin was excluded from the midb ody which contains an actin filament-rich contractile ring. Our result s indicate that beta 4 and beta 10 are functionally very similar and b oth are effective regulators of a large subset of actin filaments in l iving cells. (C) 1994 WiIey-Liss, Inc.