PHOSPHORYLATION OF PITSLRE P110 ISOFORMS ACCOMPANIES THEIR PROCESSINGBY CASPASES DURING FAS-MEDIATED CELL-DEATH

A number of cellular proteins have been identified as caspase targets during cell death, including the PITSLRE protein kinases. These target s generally fall into one of three possible categories: 1) other caspa ses, 2) proteins that are inactivated during apoptosis, and 3) protein s that are required for execution of the cell death program. However, not all proteins are cleaved by caspases during apoptosis. Why only sp ecific proteins are destined to be processed by caspases during cell d eath is currently not clear. Here we show that multiple caspase-like a ctivities are involved in the processing of the PITSLRE p110 isoforms during Fas-induced apoptosis in Jurkat T-cells. Three p110 caspase cle avage sites have been mapped to the amino-terminal domain of p110 and verified by site-directed mutagenesis. Curiously, the mutagenesis stud ies revealed that cleavage of two juxtaposed caspase sites is necessar y for the complete processing of this protein during cell death in viv o. Finally, we demonstrate that the PITSLRE p110 protein is rapidly ph osphorylated during Fas-induced apoptosis in Jurkat cells and that pho sphorylation of an aminoterminal portion of the protein may enhance ca spase cleavage in this region.