Degradation of poly(A)-binding protein in apoptotic cells and linkage to translation regulation

We have recently shown that poly(A)-binding protein (PABP) is cleaved during poliovirus and Coxsackievirus infection by viral 3Cprotease and that 3Cprotease modification of a subset of PABP can result in significant translation inhibition. During apoptosis, translation undergoes significant down-regulation that correlates with caspase-3 mediated cleavage of several translation factors, including eIF4G, 4EBP1 and eIF2alpha. The fate of PABP in apoptotic cells has not yet been examined. Here we show that PABP levels decline significantly via proteolytic degradation in apoptotic HeLa, Jurkat and MCF7 cells. The degradation of PABP correlated with translation inhibition but lagged behind cleavage of eIF4GI. In apoptotic MCF7 cells translation inhibition occurred without modification of most translation factors and correlated with PABP degradation. PABP was not cleaved during incubation with several caspases, yet caspase 3 induced weak PABP degradative activity in cells lysates. Both the caspase inhibitor zVAD and calpain inhibitors blocked PABP cleavage in vivo, while the proteosome inhibitor MG132 induced PABP degradation. Protease(s) activated during apoptosis preferentially degraded PABP associated with ribosomes and translation factors, but not PABP in other cellular compartments. The data suggest that targeted degradation of PABP contributes to translation inhibition in apoptotic cells.