Structural Analysis
Procaspase 8 (predicted)
Procaspase 8 consists of a catalytic domain and a prodomain, separated by a linker. The prodomain contains two identical Death Effector Domains (DEDs), the second of which binds to the DEDs of Fas-associated protein Death Domain (FADD), the cell surface receptor that induces casp8 dependent apoptosis [1]. This binding facilitates dimerisation of procaspase 8 and subsequent autocleavage at the three sites shown. The resultant active form of caspase 8 is therefore dimeric, the p18 and p10 subunits of the catalytic domain forming the binding pocket for BID and the prodomain being lost. This is a predicted structure for procaspase 8, so to further investigate how structure relates to function we will use the crystal structures determined independently for the DEDs and the active form.
DED
DED1 and 2 are identical in amino acid sequence, both with six α-helices. The residue responsible for FADD binding is an FL situated on helix α2, but due to the nature of the dimerization only the DED2 FL is available, and as such there is only one FADD binding domain per procaspase 8 [2]. With the crystal structure determined for the procaspase 8 DED domain [3] we show how the two FL residues are differently positioned in the structure, and the polar interactions that hold the dimer together.
Source: http://www.rcsb.org/pdb/explore/explore.do?structureId=4ZBW
Casp8
The crystal structure for active casp8 is shown here with an inhibitor covalently bound to the catalytic cysteine. The binding pocket is formed from both the p18 and p10 subunits, the catalytic site being present on the p18. The polar interactions between inhibitor, binding pocket and catalytic site are shown; these may mirror those between casp8 and its substrate BID. Casp8 cleaves BID at the sequence LQTDG [4], which then goes on to activate mitochondria dependent apoptosis [5]. The structural core of casp8 consists of 6 hydrophobic β-sheets (four from p18, two from p10) and 5 α-helices (three from p18, two from p10).
Source: http://www.rcsb.org/pdb/explore/explore.do?structureId=3KJQ
Casp8p41 (predicted)
Casp8p41 results from cleavage of procaspase 8 by HIV protease [6]. This form of casp8 does not contain the catalytic cysteine but can still initiate mitochondria-dependent apoptosis as it contains the FADD binding domain and can dimerise with regular procaspase 8 to facilitate autocleavage, producing active casp8. Casp8p41 is also thought to initiate mitochondria-independent apoptosis, perhaps by acting as a scaffold for other proteins. The FADD binding FL of the prodomain is still seen to be available for binding, but the catalytic domain is heavily truncated.
Casp8L (predicted)
Casp8L is an isoform that has a 136bp insertion between exons 8 and 9 [7], resulting in the occurrence of an early stop codon. Casp8L therefore lacks almost all the catalytic domain, though the pro-domain remains intact, as shown. As with Casp8p41, the FADD binding FL residue is still available, though unlike Casp8p41, Casp8L cannot induce autocleavage. Casp8L is hence a competitive inhibitor of procaspase 8, an inhibitor of apoptosis.
[1] Oberst, A. et al. "Inducible Dimerization And Inducible Cleavage Reveal A Requirement For Both Processes In Caspase-8 Activation". Journal of Biological Chemistry 285.22 (2010): 16632-16642. Web.
[2] Carrington, Paul E. et al. "The Structure Of FADD And Its Mode Of Interaction With Procaspase-8". Molecular Cell 22.5 (2006): 599-610. Web.
[3] Shen, Chen et al. "Crystal Structure Of The Death Effector Domains Of Caspase-8". Biochemical and Biophysical Research Communications 463.3 (2015): 297-302. Web.
[4] Li, Honglin et al. "Cleavage Of BID By Caspase 8 Mediates The Mitochondrial Damage In The Fas Pathway Of Apoptosis". Cell 94.4 (1998): 491-501. Web.
[5] Oberst, A. et al. "Inducible Dimerization And Inducible Cleavage Reveal A Requirement For Both Processes In Caspase-8 Activation". Journal of Biological Chemistry 285.22 (2010): 16632-16642. Web.
[6] Badley, Andrew D. et al. "Analysis Of HIV Protease Killing Through Caspase 8 Reveals A Novel Interaction Between Caspase 8 And Mitochondria". The Open Virology Journal 1.1 (2008): 39-46. Web.
[7] Himeji, D. "Characterization Of Caspase-8L: A Novel Isoform Of Caspase-8 That Behaves As An Inhibitor Of The Caspase Cascade". Blood 99.11 (2002): 4070-4078. Web.