1. ¹Program in Apoptosis and Cell Death Research, Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA
2. ²Division of Medicinal Chemistry and Microbiology, Faculty of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, Wroclaw 50-370, Poland
3. ³Department of Haemostasis Biochemistry, BRU, Novo Nordisk A/S, Maaloev DK-2760, Denmark

Correspondence: GS Salvesen, Program in Apoptosis and Cell Death Research, Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA. Tel: +1 858 646 3114; Fax: +1 858 646 3199; E-mail: gsalvesen@burnham.org

Received 6 September 2007; Revised 23 January 2008; Accepted 25 January 2008; Published online 29 February 2008.

Abstract

Drosophila Nedd2-like caspase (DRONC), an initiator caspase in Drosophila melanogaster and ortholog of human caspase-9, is cleaved during its activation in vitro and in vivo. We show that, in contrast to conclusions from previous studies, cleavage is neither necessary nor sufficient for DRONC activation. Instead, our data suggest that DRONC is activated by dimerization, a mechanism used by its counterparts in humans. Subsequent cleavage at Glu352 stabilizes the active dimer. Since cleavage is at a Glu residue, it has been proposed that DRONC is a dual Asp- and Glu-specific caspase. We used positional-scanning peptide libraries to define the P1–P4 peptide sequence preferences of DRONC, and show that it is indeed equally active on optimized tetrapeptides containing either Asp or Glu in P1. Furthermore, mutagenesis reveals that Asp and Glu residues are equally tolerated at the primary autoprocessing site of DRONC itself. However, when its specificity is tested on a natural substrate, the Drosophila executioner caspase DRICE, a clear preference for Asp emerges. The formerly proposed Glu preference is thus incorrect. DRONC does not differentiate between Asp and Glu in poor substrates, but prefers Asp when tested on a good substrate.