A highly conserved family of chromosomal proteins are caspase substrates during apoptosis

Apoptosis, or programmed cell death (PCD), is an essential component of embryogenesis, immune system function, and tissue homeostasis in eukaryotic organisms. This process is mediated by, and absolutely requires, the activity of caspases, cysteine endoproteases that recognize specific tetrapeptide sequences having an aspartate residue at the P1 location of the cleavage site. A morphological hallmark of this process is the appearance of condensed and fragmented nuclei. However, the mechanism bridging caspase activation with apoptotic chromatin condensation is unknown. In this report, we demonstrate that FIN16, a mouse protein encoded by the fibroblast growth factor-inducible cDNA 16 (FIN16), is a novel caspase substrate. A BLAST search of the nonredundant GenBank database and subsequent sequence alignment revealed high homology between FIN16 and the carboxy terminal domains of structural maintenance of chromosomes (SMC) family proteins, including the Saccharomyces cerevisiae SMC2, Xenopus laevis XCAP-E condensin, and Gallus gallus Scll chromosomal scaffold proteins. Importantly, all of these proteins contain a potential caspase-3 cleavage sequence, DEVD. In vitro, purified caspase-3 cleaved radiolabelled FIN16 generating two fragments, one of which was probably generated by cleavage after D305, as expected. However, FIN16 did not serve as a substrate for caspase-8, but was cleaved by granzyme B at a different site than D305. Furthermore, apoptotic cytosolic extracts from Fas/CD95- and staurosporine-activated cells were also able to process the FIN16 protein. This was fully inhibited by the tetrapeptide inhibitors Z-VAD-FMK and Z-DEVD-FMK, confirming the involvement of caspases. The presence of an evolutionarily conserved caspase consensus sequence DEV/ID in SMC proteins further implicates the caspase-mediated cleavage of these proteins as a potential mechanism of apoptotic chromatin condensation and/or destruction of chromosomal structure.