Biomedical Engineering Reference
In-Depth Information
molecules of 105 kDa (p105) and 100 kDa (p100), respectively. The N-terminal
regions of p105 and p100 constitute the RHD of p50 and p52 while the C-termini
of each molecule contain multiple copies of ankyrin repeat sequences, which are
found in I
B family members (see the next section). Indeed, both p105 and p100
inhibit the nuclear localization and transcriptional activity of NF-
κ
B/Rel proteins
with which they dimerize. Generation of p50 from p105 and p52 from p100, although
κ
superficially similar, occurs by distinct processing mechanisms (see
Chapter 4
).
Signal-induced processing of p100 to release transcriptionally active p52/RelB
dimers occurs following inducible phosphorylation of specific C-terminal serine
residues by IKK
, leading to subsequent ubiquitination and incomplete processing
by the proteasome. In contrast, the processing of p105 appears to occur constitutively,
leading to the generation of a fixed ratio of p105 to p50 in an individual cell. It has
been proposed that the constitutive processing of p105 occurs through a cotransla-
tional mechanism, although details about the process and its regulation remain
sketchy. The mechanism by which inducible p100 processing occurs is also poorly
defined, and it is clear that much more work remains to be done in answering
fundamental questions about these important events in the NF-
α
κ
B activation pathway.
1.3
I
κ
B PROTEINS
During the original purification of NF-
κ
B, it was discovered that DNA-binding of
cytoplasmic NF-
B could be induced by treatment of cytosolic extracts with disso-
ciating agents such as sodium deoxycholate or formamide [6,7]. This led to the
finding that two distinct proteins associated with NF-
κ
κ
B and inhibited its activity
in nonstimulated cells [8-10]. These proteins, termed I
, had molecular
weights of 37 and 43 kDa respectively, and purified preparations of these proteins
could both specifically and reversibly inhibit DNA-binding of NF-
κ
B
α
and I
κ
B
β
κ
B, and disasso-
ciate DNA-bound NF-
B
in vitro
. Functional domains of these inhibitory molecules
became apparent after their molecular cloning, which revealed that they contain
repeated sequences of 30 to 33 amino acids known as ankyrin repeats [11,12]. As
described above, the p50 and p52 precursor molecules p105 and p100 also contain
ankyrin repeats in their C-terminal regions and, prior to processing, are capable of
inhibiting NF-
κ
B molecules have subsequently been found to
contain between three and seven ankyrin repeats. To date, with the inclusion of I
κ
B activity. All I
κ
κ
B
ζ
,
eight structurally related members of the mammalian I
κ
B family have been cloned.
The I
κ
B family includes I
κ
B
α
, I
κ
B
β
, I
κ
B
ε
, BCL-3, I
κ
B
ζ
, I
κ
B
γ
, p105, and p100
(
Chapter 2
). Unlike the other I
κ
B proteins, I
κ
B
γ
is not the product of a distinct gene
but is a 70 kDa protein containing the C-terminal domain of p105 that arises from
a unique transcript of p105 and is expressed only in certain murine lymphoid cells.
There are two other less well-studied I
B-
related protein), which is a 52 kDa molecule that appears to be preferentially
expressed in epithelial cells; however, the
in vivo
regulatory function of this molecule
is not known. An additional I
κ
B isoforms. The first is I
κ
BR (for I
κ
B-NS, was reported to be important
in negative selection of thymocytes and also in suppressing inflammatory responses
under certain situations.
κ
B-like protein, I
κ
