Protein C-terminal enzymatic labeling identifies novel caspase cleavages during the apoptosis of multiple myeloma cells induced by kinase inhibition.

Caspase activation and proteolytic cleavages are the major events in the early stage of apoptosis. Identification of protein substrates cleaved by caspases will reveal the occurrence of the early events in the apoptotic process and may provide potential drug targets for cancer therapy. Although several N-terminal MS-based proteomic approaches have been developed to identify proteolytic cleavages, these methods have their inherent drawbacks. Here we apply a previously developed proteomic approach, protein C-terminal enzymatic labeling (ProC-TEL), to identify caspase cleavage events occurring in the early stage of the apoptosis of a myeloma cell line induced by kinase inhibition. Both previously identified and novel caspase cleavage sites are detected and the reduction of the expression level of several proteins is confirmed biochemically upon kinase inhibition although the current ProC-TEL procedure is not fully optimized to provide peptide identifications comparable to N-terminal labeling approaches. The identified cleaved proteins form a complex interaction network with central hubs determining morphological changes during the apoptosis. Sequence analyses show that some ProC-TEL identified caspase cleavage events are unidentifiable when traditional N-terminomic approaches are utilized. This work demonstrates that ProC-TEL is a complementary approach to the N-terminomics for the identification of proteolytic cleavage events such as caspase cleavages in signaling pathways.