Biomedical Engineering Reference
In-Depth Information
It is generally believed that different I
κ
B molecules preferentially inhibit distinct
subsets of NF-
κ
B/Rel protein dimers (
Chapter 2
). For example, both I
κ
B
α
and I
κ
B
β
strongly inhibit complexes containing c-Rel and p65, whereas BCL-3 and I
only
bind homodimers of p50 or p52. Specific inhibitory property has also been reported
for I
κ
B
ζ
, which appears to only bind to c-Rel, p65, and their respective homodimers.
By comparison, p100 and p105 exhibit little specificity, and they have been reported
to form inhibitory complexes with p50, p52, p65, and c-Rel. Interestingly, the I
κ
B
ε
B
molecules most often found associated with RelB are p100, consistent with the
observation that processing of p100 in response to the “alternative” pathway leads
to release of p52:RelB dimers. Inhibition of NF-
κ
B molecules
occurs via protein-protein interactions between the ankyrin repeats of I
κ
B/Rel proteins by I
κ
κ
B and
regions of the RHD in NF-
κ
B. With the exception of BCL-3, I
κ
B
ζ,
and an unphos-
phorylated form of I
κ
B-
β
, this interaction enables the I
κ
B proteins to mask the NLS
and prevent nuclear translocation of NF-
B/Rel protein dimers (Chapter 2).
In addition to the ankyrin repeats, which occur in the interior of I
κ
B proteins, the
N- and C-terminal domains of each molecule contain important structural and func-
tional domains. The N-terminal regions contain the signal-responsive sites of phos-
phorylation in I
κ
κ
B
α
, I
κ
B
β,
and I
κ
B
ε
. Deletion of the C-terminal domain blocks the
ability of I
κ
Bs to prevent DNA-binding of NF-
κ
B and to dissociate DNA-bound
NF-
B proteins contain an acidic and Thr-rich
proline, glutamic acid, serine, and threonine (PEST) sequence, which is believed to
play a role in stabilization of the molecule. Deletion studies have demonstrated that
removal of the PEST domain can protect I
κ
B dimers. Within their C-termini, I
κ
κ
B
α
from degradation induced during
NF-
B activation (see below) and basal phosphorylation of sites within this domain
(that is, by casein kinase II) has been implicated in regulating the constitutive turnover
of I
κ
κ
B molecules in unstimulated cells.
1.4
I
κ
B KINASE COMPLEX
Degradation of I
B is a tightly regulated event that is initiated upon specific phos-
phorylation by activated IKK. The IKK activity in cells can be purified as a 700 to
900 kDa complex and has been shown to contain two kinase subunits, IKK
κ
α
(IKK1)
and IKK
β
(IKK2), and a regulatory subunit NEMO (NF-
κ
B essential modifier) or
IKK
γ
(see
Chapter 3
). In the classical NF-
κ
B signaling pathway, IKK
β
is both
necessary and sufficient for phosphorylation of I
κ
B
α
on Ser
32
and Ser
36
, and I
κ
B
β
on Ser
19
and Ser
23
. The role of IKK
in the classical pathway is unclear, although
recent studies suggest it may regulate gene expression in the nucleus by modifying
the phosphorylation status of histones. The alternative pathway, however, depends
only on the IKK
α
subunit, which functions by phosphorylating p100 and causes its
inducible processing to p52 [13]. The alternative pathway is activated in response
to a subset of NF-
α
κ
B inducers including LT
β
and B cell activating factor (BAFF).
B proteins are recognized and ubiquitinated by
members of the Skp1-Culin-Roc1/Rbx1/Hrt-1-F-box (SCF or SCRF) family of ubiq-
Upon phosphorylation by IKKs, I
κ
uitin ligases (see
Chapter 4
). The ubiquitinated I
κ
B proteins are then degraded
rapidly by the proteasome. Despite the enormous interest in the IKK complex, there
