Crystal Structure of a Procaspase-7 Zymogen

Abstract

Apoptosis is primarily executed by active caspases, which are derived from the inactive procaspase zymogens through proteolytic cleavage. Here we report the crystal structures of a caspase zymogen, procaspase-7, and an active caspase-7 without any bound inhibitors. Compared to the inhibitor-bound caspase-7, procaspase-7 zymogen exhibits significant structural differences surrounding the catalytic cleft, which precludes the formation of a productive conformation. Proteolytic cleavage between the large and small subunits allows rearrangement of essential loops in the active site, priming active caspase-7 for inhibitor/substrate binding. Strikingly, binding by inhibitors causes a 180 degrees flipping of the N terminus in the small subunit, which interacts with and stabilizes the catalytic cleft. These analyses reveal the structural mechanisms of caspase activation and demonstrate that the inhibitor/substrate binding is a process of induced fit.

Human Caspase-7 Activity and Regulation by Its N-terminal Peptide

“…The former possibility suggests that the N-peptide alters the conformation of the zymogen in a subtle manner. However, the atomic resolution structures available for full-length caspase-7 or pro-caspase-7 do not reveal density for residues prior to Ser 47 , either because there is no defined structure or because of mobility with respect to the fixed bulk of the catalytic domains in the crystals (4,6,39). In addition, there are no contacts visible between the N-peptide and the catalytic domain.…”

“…Cleavage within the linker region of caspase-7 is strictly required for its activation (4,6,21,22). This region contains 2 cleavage sites that could each potentially result in activation of the zymogen (Fig.…”

Caspase activation – stepping on the gas or releasing the brakes? Lessons from humans and flies

“…At cytosolic concentration in human cells, the caspase 3 Pop et al, 2001) and 7 zymogens are already dimers (Boatright et al, 2003), but cleavage within their respective linker segments is required for activation (Chai et al, 2001b;Riedl et al, 2001a). The same reordering of catalytic and substrate binding residues occurs in caspase 7 as seen in caspase 9, so the fundamental mechanism of zymogen activation is equivalent.…”

“…They are activated by limited proteolysis within their interdomain linker carried out by an initiator caspase, and occasionally by other proteases under specific circumstances. The crystal structures of zymogen caspase 7, active caspase 7, and inhibitor-bound caspase 7 serve as a model with which to rationalize the apparent conflict between the cleavage mechanism for executioner caspase activation, and the dimerization mechanism for apical caspase activation (Wei et al, 2000;Chai et al, 2001b;Riedl et al, 2001a).…”

Oligomerization and activation of caspase-9, induced by Apaf-1 CARD

“…Once activated, the effector caspases cleave numerous cellular targets, ultimately leading to cell death. Although the activation of effector caspases is structurally characterized (10), how the initiator caspases are activated and regulated remains poorly understood.…”