Biomedical Engineering Reference
In-Depth Information
involves casein kinase 2 (CK2) [64]. UV activates CK2 via p38 MAP kinase, which
acts as an allosteric CK2 regulator [65]. Activated CK2 then directly phosphorylates
I
κ
B
α
on a cluster of serines in the C-terminal PEST region, triggering its degradation
by the proteasome and activating NF-
κ
B (
Figure 4.4
)
[64]. I
κ
B
α
can therefore be
targeted for ubiquitin-dependent proteasomal degradation via two signaling path-
ways, which regulate distinct target phosphorylation sites on I
κ
B
α
. The activation
of NF-
B by the anticancer agent doxorubicin appears to be controlled by a different
IKK-independent pathway. This induces I
κ
degradation independently of phos-
phorylation of its N-terminus or PEST region and requires PI 3-kinase activity [66].
κ
B
α
4.5
PROTEOLYSIS OF THE p105 AND p100
PRECURSOR PROTEINS
B2)
are synthesized as large inactive precursor proteins of 105 (p105) and 100 (p100)
In contrast to the other members of the Rel family, p50 (NF-
κ
B1) and p52 (NF-
κ
kDa, respectively (
Chapter 2
) [67]. Both precursors function as I
κ
B proteins, which
retain associated Rel subunits in the cytoplasm. Generation of the mature transcrip-
tion factors, p50 and p52, from p105 and p100, respectively, involves ubiquitination
and partial proteolysis by the proteasome (
Figure 4.5
).
These proteolytic events,
known as processing, remove their C-terminal halves, which include the I
κ
B-like
ankyrin repeat region.
4.5.1
NF-
κ
B1 p105/p50
Processing of p105 to generate p50 occurs constitutively, and p50 generation is not
usually modified following agonist stimulation [68]. However, cell stimulation with
TNF
or lipopolysaccharide (LPS) increases the proteolysis of p105, leading pre-
dominantly to its complete degradation [10,69,70]. This releases associated Rel
subunits to translocate from the cytoplasm into the nucleus and modulate expression
of genes that regulate inflammatory responses [71].
α
4.5.1.1
Processing of NF-
κ
B1 p105 to p50
The partial proteolysis of p105 by the proteasome to generate p50 is very unusual,
as the proteasome normally completely degrades proteins [72]. Consequently, the
mechanism by which p50 is produced from p105 has been intensely investigated. It
has been suggested that p105 is processed cotranslationally by the proteasome and
that synthesis of complete p105 molecules is not required for p50 generation [73].
However, several laboratories have demonstrated a clear precursor-product relation-
ship between p105 and p50 in pulse-chase experiments [74,75,76,77] and it remains
uncertain whether cotranslational p105 processing is physiologically important.
The central glycine-rich region (GRR), located between the RHD and ankyrin
repeats, was the first motif in p105 shown to be required for processing to p50; a
p105 deletion mutant lacking the GRR is unable to generate p50 [78], although p105
ubiquitination is not affected [79]. A related glycine-alanine repeat domain of
EBNA1 (EBV nuclear antigen 1) blocks the proteolysis of EBNA1 by the proteasome
[80]. Furthermore, insertion of the EBNA1 glycine-alanine repeat into different
