MICROFILAMENT REORGANIZATION DURING APOPTOSIS - THE ROLE OF GAS2, A POSSIBLE SUBSTRATE FOR ICE-LIKE PROTEASES

Gas2, a component of the microfilament system, belongs to the class of gas genes whose expression is induced at growth arrest, After serum o r growth factor addition to quiescent NIH 3T3 cells, Gas2 is hyperphos phorylated and relocalized at the membrane ruffles. By overexpressing gas2wt and a series of deletion mutants of the C-terminal region, we h ave analysed its role in the organization of the actin cytoskeleton in different cell lines, Overexpression of Gas2 deleted at its C-termina l region (Delta 276-314 and Delta 236-314), but not its wild-type form , induces dramatic changes in the actin cytoskeleton and cell morpholo gy, These effects are not-due to interference of the deleted forms wit h the endogenous Gas2wt function but could be ascribed to a gain of fu nction. We demonstrate that during apoptosis the C-terminal domain of Gas2 is removed by proteolytic cleavage, resulting in a protein that i s similar in size to the described Delta 276-314. Moreover, by using i n vitro mutagenesis, we also demonstrate that the proteolytic processi ng of Gas2 during apoptosis is dependent on an aspartic acid residue a t position 279. The evidence accumulated here could thus represent a f irst example of a mechanism linking apoptosis with the co-ordinated mi crofilament-dependent cell shape changes, as possibly mediated by an i nterleukin-1 beta-converting enzyme (ICE)-like dependent proteolytic c leavage of the Gas2 protein.