ACTIN IS CLEAVED DURING CONSTITUTIVE APOPTOSIS

Proteases play an important role in the programme of cell death by apo ptosis but little is known of the substrates cleaved, particularly in constitutive models of this type of cell death. Neutrophils spontaneou sly undergo apoptosis in culture without requiring external stimuli. D uring this process we found biochemical and immunochemical evidence fo r the cleavage of membrane-associated actin, a component of the cytosk eleton that links polymerized actin to the plasma membrane. Cleavage o ccurred at a single site at the N-terminus, between residues Val(43)-M et(44), a site devoid of a consensus motif for cleavage by cysteine pr oteases of the interleukin-1 beta-converting enzyme (ICE)-family. Wher eas actin cleavage and nuclear/cell surface markers of apoptosis were co-ordinately diminished by zVAD-fmk, an inhibitor of the ICE-like fam ily of proteases, only acetyl-leucyl-leucylnormethional, an inhibitor of calpains, was capable of completely inhibiting actin cleavage. Our results suggest that actin is not a direct substrate for the ICE-like family of proteases. By disabling the cytoskeleton, actin cleavage may be an important component in the capacity of apoptosis to reduce the injurious potential of neutrophils.