Nuclear translocation of cleaved LyGDI dissociated from Rho and Rac during Trp53-dependent ionizing radiation-induced apoptosis of thymus cells in vitro.

LyGDI inhibits the dissociation of GDP from Rho family GTPases and is found in abundance in hematopoietic cells. Here we report truncation of LyGDI after irradiation in mouse 3SB thymus cells. A 21-kDa fragment of LyGDI, resulting from activated caspase 3-induced cleavage at an N-terminal consensus site following the Asp(18) residue, accumulated at peak quantities between 5 and 12 h after irradiation. Cleavage of LyGDI was inhibited by the caspase inhibitor benzoyloxycarbonyl-Val-Asp-fluoromethylketone. Subcellular fractionation and immunofluorescence revealed the truncated 21-kDa fragment of LyGDI within the nuclear fraction of irradiated 3SB cells, whereas full-length LyGDI was found only in the cytoplasmic fraction. Truncated LyGDI within the nucleus had no association with the Rho family proteins RhoA and Rac1, since these proteins were observed only in the cytoplasmic fractions. These data demonstrate that regulation of Rho family GTPases by LyGDI is disrupted during apoptosis, suggesting that fragmentation of LyGDI implicates the transmission of a signal from the cytoplasm to the nucleus during Trp53-dependent apoptosis of thymus cells after irradiation.