Role of IAP-proteins in apoptosis
Apoptotic response is activated through either the
intrinsic or the extrinsic
pathway, depending on the origin of the death stimuli.
The extrinsic pathway is initiated by death receptors,
such as FasR (TNF receptor superfamily, member 6) and
TNFR1 (tumor necrosis factor receptor superfamily, member
1A). Activation of the FasR and TNFR1
by its cognate ligand results in the recruitment and activation of
pro-caspase-8 (caspase-8). Active
caspase-8 subsequently cleaves and activates pro-caspase-3
(caspase-3) and pro-caspase-7
(caspase-7). Activation of
caspase-3 leads to the ultimate demise of the cell.
Caspase-8 and caspase-9 are
often referred to as initiator caspases, whereas caspase-3
and caspase-7 are considered as effector caspases .
The intrinsic, or mitochondrial apoptotic pathway is
activated under stress and by some developmental apoptotic triggers. This pathway is
thought to be triggered by translocation of proteins from the pro-apoptotic Bcl-2 family
member, such as Bax, into the mitochondria. This event
triggers the release of mitochondrial proteins, such as cytochrome c,
Smac/DIABLO and Omi/HtrA2 (high-temperature
requirement protein A2), and some others .
Bax pemeabilization of mitochondria also induces release
of Apoptosis-inducing factor, mitochondrion-associated 1
(Aif), which translocates to the nucleus and induces DNA
fragmentation in a caspase-independent manner .
The extrinsic pathway crosstalks to the
intrinsic pathway via caspase-8-mediated cleavage of
Bid (a BH3-ONLY member of the BCL2 FAMILY of proteins).
After cleavage by caspase-8,
tBid translocates to the mitochondrion and triggers the
intrinsic pathway .
Thereby, the mitochondrial pathway serves as an amplification loop caspases. This
amplification occur by two pathways - via activation of
caspase-9 and via inhibition of baculoviral IAP
repeat-containing proteins (IAPs).
Following its release from mitochondria, cytochrome c
binds to and activates protein Apaf-1 in the cytoplasm,
followed by binding of Apaf-1 to ATP/dATP and to formation
of the apoptosome. Apoptosome mediates activation of
caspase-9, thereby triggering a cascade of caspase
The baculoviral IAP repeat-containing proteins (IAPs)
c-IAP2, NAIP and survivin)
inhibit certain caspases (caspase-3,
caspase-7 and caspase-9) by
direct binding or by ubiquitylation. Mitochondrial protein
Smac/DIABLO, on its release from mitochondria, binds
c-IAP2, and survivin in a
manner that displaces caspases. So, Smac/DIABLO is a
negative regulator of IAPs, and is therefore an apoptosis-enhancing molecule .
Cleavage of caspase-9 is not required for activation but
is necessary for inactivation of caspase-9 by
XIAP. XIAP sequesters
caspase-9 in a monomeric state, which serves to prevent
catalytic activity .
XIAP regulates activity of
Aif during apoptosis by its ubiquitination .
Serine protease HtrA2/OMI also inhibits IAP-proteins by
binding or by cleavage .
Peptides derived from the C termini of human Presenilins
modulate the protease and apoptotic activities of Omi/HtrA2
by binding to its PDZ domain .
In response to cell death stimulation, survivin
associates with XIAP via conserved baculovirus IAP repeats.
Formation of a survivin-XIAP complex promotes increased
XIAP stability against ubiquitination/proteasomal
destruction and synergistic inhibition of apoptosis .
However, XIAP itself can be a target for caspases and
that cleavage destroys its antiapoptotic function .
Apoptosis couples to surveillance mechanisms and survivin
is a candidate molecule for the interface between apoptosis and cell cycle.
Survivin selectively expressed at G2/M and
localized to mitotic spindle microtubules. Survivin as a
mitotic substrate of CDK1-cyclin
B1 and survivin phosphorylation on Thr34 was required to
interaction with initiator caspase-9 .
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