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. 1999 Apr 15;18(8):2031–2039. doi: 10.1093/emboj/18.8.2031

Crystal structure of baculovirus P35: role of a novel reactive site loop in apoptotic caspase inhibition.

A J Fisher 1, W d Cruz 1, S J Zoog 1, C L Schneider 1, P D Friesen 1
PMCID: PMC1171287  PMID: 10205157

Abstract

The aspartate-specific caspases are critical protease effectors of programmed cell death and consequently represent important targets for apoptotic intervention. Baculovirus P35 is a potent substrate inhibitor of metazoan caspases, a property that accounts for its unique effectiveness in preventing apoptosis in phylogenetically diverse organisms. Here we report the 2.2 A resolution crystal structure of P35, the first structure of a protein inhibitor of the death caspases. The P35 monomer possesses a solvent-exposed loop that projects from the protein's main beta-sheet core and positions the requisite aspartate cleavage site at the loop's apex. Distortion or destabilization of this reactive site loop by site-directed mutagenesis converted P35 to an efficient substrate which, unlike wild-type P35, failed to interact stably with the target caspase or block protease activity. Thus, cleavage alone is insufficient for caspase inhibition. These data are consistent with a new model wherein the P35 reactive site loop participates in a unique multi-step mechanism in which the spatial orientation of the loop with respect to the P35 core determines post-cleavage association and stoichiometric inhibition of target caspases.

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