MOLECULAR-CLONING AND PRO-APOPTOTIC ACTIVITY OF ICE(REL)II AND ICE(REL)III, MEMBERS OF THE ICE/CED-3 FAMILY OF CYSTEINE PROTEASES

Cysteine proteases related to mammalian interleukin-1 beta-converting enzyme (ICE) and the nematode cell death abnormal ced-3 gene product have been implicated in the effector mechanism of apoptotic cell death. Two novel members of this new family of ICE/CED-3-related proteases, designated ICE(rel)-II and ICE(rel)-III, were cloned from human monocytic cells. Both were highly homologous to human ICE (52% identical) and CED-3(25% identical) and both contained the absolutely conserved pentapeptide sequence Gln-Ala-Cys-Arg-Asp containing the catalytic cysteine residue. Other structural motifs that were comparable with ICE suggest that ICE(rel)-II and ICE(rel)-III are also synthesized as larger proenzymes which are proteolytically processed to form heterodimeric active enzymes. Pro-interleukin-1 beta processing activity could not be detected in cells transfected with ICE(rel)-II or ICE(rel)-III, but pro-domain-less truncated forms of ICE(rel)-II and ICE(rel)-III were capable of effectively inducing fibroblast apoptosis. ICE(rel)-II and ICE(rel)-III may, therefore, participate in proteolytic events culminating in the apoptotic death of human cells.