The holo-apoptosome: activation of procaspase-9 and interactions with caspase-3

Abstract

Activation of procaspase-9 on the apoptosome is a pivotal step in the intrinsic cell death pathway. We now provide further evidence that caspase recruitment domains of pc-9 and Apaf-1 form a CARD-CARD disk that is flexibly tethered to the apoptosome. In addition, a 3D reconstruction of the pc-9 apoptosome was calculated without symmetry restraints. In this structure, p20 and p10 catalytic domains of a single pc-9 interact with nucleotide binding domains of adjacent Apaf-1 subunits. Together, disk assembly and pc-9 binding create an asymmetric proteolysis machine. We also show that CARD-p20 and p20-p10 linkers play important roles in pc-9 activation. Based on the data, we propose a proximity-induced association model for pc-9 activation on the apoptosome. We also show that pc-9 and caspase-3 have overlapping binding sites on the central hub. These binding sites may play a role in pc-3 activation and could allow the formation of hybrid apoptosomes with pc-9 and caspase-3 proteolytic activities.

Figure 1. A CARD-CARD disk is present on the heptameric Apaf-1 platform

A. Eight representative class averages are shown of double apoptosomes. The number of particles in each class is shown in the lower left corner of each panel. B. Top views are shown of rotary metal shadowed, pc-9 CARD apoptosomes. A strongly contrasted CARD-CARD disk is observed (white arrows). C. Top views are shown of ground state apoptosomes. White arrows indicate weakly-contrasted rings in some particles that may be due to Apaf-1 CARDs. See also Figure S1.

Figure 2. Apaf-1 CARDs are flexibly-linked to the central hub in the human apoptosome

A. A thrombin site was inserted into the CARD-NBD linker of Apaf-1 (marked with an asterisk) to create Apaf-1tb. B. Purified Apaf-1 with a thrombin site was assembled and run on a glycerol gradient. The gel profile shows that Apaf-1tb forms apoptosomes which run in fractions 6-8, while excess cytochrome c remains in fractions 1-3. C. Thrombin cleavage of Apaf-1tb apoptosomes released CARDs, which stayed at the top (fractions 1-3), while the platform region migrated into the gradient (fractions 9-12). D. Thrombin cleavage of the Apaf-1 monomer shows that the CARD-NBD linker is accessible. E. A molecular model is shown for flexibly-tethered CARDs on the Apaf-1 platform and depicts the effects of thrombin. See also Figure S2.

Figure 3. The CARD-p20 linker plays a critical role in pc-9 activation

A. A cartoon is shown of pc-9 domains and linkers. A thrombin site and various mutations are indicated on the expanded linker sequences. B. Proteolytic activities (rates) of various pc-9 mutants with D315A, either with or without the apoptosome, are shown as a histogram. Values for thrombin treated samples are also shown. Error bars indicate the standard error for triplicate measurements. C. A histogram is shown of normalized proteolytic activity for pc-9 and pc-9Δ30L, without and with the apoptosome (blue and maroon bars, respectively). D. Time courses are shown for the proteolytic activity of mutant pc-9t apoptosomes in PB with pc-3 (C163A) as substrate. Proteolytic activity generates the p20 and p10 subunits of caspase-3. E. (left) Top and side views are shown of a surface-rendered 3D map from pc-9 CARD apoptosomes assembled with pc-9tΔ30L and treated with thrombin. The CARD-CARD disk is shown in magenta and the platform is colored blue. (right) A side view is shown of the 3D map. See also Figure S3.

Figure 4. Properties of the pc-9 apoptosome

A. Time courses are shown for pc-3 cleavage by pc9 apoptosomes assembled with pc-9t (D315A), pc-9t wt, pc-9 (D315A) and pc-9 wt. B. Wild type pc-9 molecules protect against thrombin cleavage in the CARD-NBD linker within Apaf-1tb apoptosomes. (Left) pc-9 apoptosomes were run on a glycerol gradient after thrombinolysis. (Right) Apoptosomes with bound pc-9 (D315A) are not protected against thrombinolysis of the CARD-NBD linker. C. Complexes were probed directly with thrombin and the proteins were run on an 8% gel with SDS PAGE to enhance the resolution of the bands. See also Figure S4.

Figure 5. Asymmetric structure of the pc-9 apoptosome

A. A top view is shown of the pc-9 apoptosome refined without symmetry restraints. A single copy of the pc-9 catalytic domains (gold) is present on the central hub. The platform is colored blue and a dashed circle indicates the expected size of the disk. B. The view in panel A is shown with a model of the platform docked into a semi-transparent rendered surface map. Note that the CARD-CARD disk (magenta) is displaced laterally relative to NBDs (dark blue) in the hub. C. A side view is shown of the pc-9 apoptosome with the catalytic domains eclipsing the disk. D. A front view is shown with the complex rotated ~77° from panel C. E. Views from the top, inside surface and front are shown of the pc-9 catalytic domains within a transparent rendering of the map. The “inside” view shows the pc-9 catalytic domains as viewed from the disk. A short ordered region of the CARD-p20 linker is shown in pink. Unfilled density is marked with an asterisk (*) and may represent a region that is formed by the CARD-p20 linker, which has not been modeled. The α8′ helix belongs to an adjacent NBD. See also Figures S5 and S6.

Figure 6. Asymmetric structure of the pc-9 CARD apoptosome

A. A top view is shown of the pc-9 CARD apoptosome. The acentric disk is shown in magenta and the platform region is rendered in blue. A dashed circle shows the estimated diameter of the disk. B. A top view is shown of the asymmetric reconstruction with a model of the platform region docked into the density. The disk is located off-center relative to the platform. C. A side view of the pc-9 CARD apoptosome shows the tilted nature of the disk. D. For comparison, a similar view to panel C is shown of the pc-9 apoptosome. The acentric disk shows a similar tilt.

Figure 7. Activation of pc-3 on the human apoptosome

A. Procaspase-3 (C163A) was added to pc-9t (D315A) apoptosomes and the cleavage reaction was continued for ~60 min. Complexes were then treated with thrombin and run on a glycerol gradient. Activated caspase-3 is bound to pc-9 CARD apoptosomes, as shown by the p20 subunit in fractions 8-12. The p10 subunit and cytochrome c overlap on this gel. Procaspase-3 and some active caspase-3 remained at the top, along with released pc-9 catalytic domains (fractions 1-3). B. Procaspase-9t (D315A) apoptosomes were treated with thrombin in LSB. Procaspase-3 (C163A) was then added for 60 min and proteins were fractionated on a glycerol gradient in PB. Unprocessed pc-3 dimers streaked into the gradient (fractions 1-6). C. (Left) Apoptosomes with pc-9t wt were run on a glycerol gradient. Bacterially cleaved p35 subunits of pc-9, containing the CARD and p20 domain, co-migrated with the apoptosome. The p10 subunit overlaps cytochrome c on the gel. (Middle) Thrombin treatment of pc-9t wt apoptosomes released the catalytic domains. (Right) Procaspase-3 was added to a similar pc-9t wt complex and treated with thrombin after a 60 min pre-incubation to release pc-9 catalytic domains. The resulting caspase-3/pc-9 CARD apoptosome peaked in fractions 8 and 9. D. Activated caspase-3 (C163A) protects pc-9 apoptosomes against thrombinolysis of a clip site in the CARD-NBD linker. (Top left) Pre-incubation for 10 min does not provide protection against cleavage. (Top right) Pre-incubation with pc-3 for 60 min protects the CARD-p20 linker against thrombinolysis, due to the formation of active caspase-3 dimers. (Bottom) pc-9 apoptosomes without and with pc-3 (60 min) were treated with thrombin and run at various loadings on the same 8% gel with Apaf-1tb apoptosomes as a control. E. The proteolytic activity of pc-9 apoptosomes was measured with Ac-LEHD-AFC and compared to complexes that contained pc-3 (C163A) added at time zero or to complexes that were pre-incubated with pc-3 for 60 min. Three pc-9 zymogens were tested: left- pc9t (D315A), middle- pc-9t wt and right- pc-9. Error bars indicate the standard error for three measurements. F. Controls were run for pc-9 apoptosome samples, including plus/minus apoptosomes and plus/minus thrombin, which cleaves the pc-9t linker releasing the catalytic domains. left - pc9t (D315A), middle- pc-9t wt and right- pc-9. The plots are normalized to 100 for pc-9t (D315A) apoptosomes.

Figure 8. A sequential path for pc-9 and pc-3 activation on the human apoptosome

The Apaf-1 platform with bound cytochrome c is shown in grey. Apaf-1 CARDs are colored light green, the acentric CARD-CARD disk is colored in magenta and subunits of pc-9 (gold) and pc-3 (blue) are color coded to indicate whether they are active or inactive. For clarity, only five pc-9 molecules are shown bound to the apoptosome. The assembled disk is drawn to scale and is acentric relative to the central hub, which may block some pc-9 and caspase-3 binding sites. Binding sites for caspase-3 are shown schematically.