title: Crystal structure of baculovirus P35: role of a novel reactive site loop in apoptotic caspase inhibition.
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.
authors: Department of Chemistry and Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA. fisher@chem.ucdavis.edu
authors: A J
authors: AJ
authors: Fisher
authors: W d
authors: Wd
authors: Cruz
authors: S J
authors: SJ
authors: Zoog
authors: C L
authors: CL
authors: Schneider
authors: P D
authors: PD
authors: Friesen
mesh_terms: D017209:Apoptosis; D001224:Aspartic Acid / Q000378:metabolism; D001483:Base Sequence; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D018360:Crystallography, X-Ray; D017931:DNA Primers; D051034:Inhibitor of Apoptosis Proteins; D008958:Models, Molecular; D017924:Nucleopolyhedroviruses / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D014764:Viral Proteins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D017931:DNA Primers; D051034:Inhibitor of Apoptosis Proteins; D014764:Viral Proteins; C086430:inhibitor of apoptosis, Nucleopolyhedrovirus; D001224:Aspartic Acid; D053148:Caspase 3; D020169:Caspases