Biomedical Engineering Reference
In-Depth Information
with
TrCP independently of phosphorylation and is not ubiquitinated or degraded,
appearing to act as a pseudosubstrate for SCF
βTrCP
[46]. hnRNP-U has no obvious
β
β
TrCP binding site and likely binds via a nonphosphorylated peptide rich in repetitive
acidic residues [46,47]. This acidic repeat may substitute for the phosphodependent
interaction of the canonical destruction motif or maintain electrostatic interactions
with the surface arginine residues of the
β
TrCP binding pocket [23]. The low affinity
interaction of
TrCP with hnRNP-U [46] may function as a safety mechanism,
preventing weak association of
β
β
TrCP with irrelevant proteins. True substrates, such
as phosphorylated I
TrCP with high affinity, may displace hnRNP-U
from SCF
βTrCP
, facilitating their specific ubiquitination and degradation.
β
κ
B, which bind
β
TrCP exists in two isoforms, 1 and 2, encoded by distinct genes [48]. The
pocket structures of the two isoforms are very similar, suggesting that
TrCP 1 and
2 may have identical targets and functions. However, analyses of cells from two
different
β
TrCP1
knockout mouse strains raise the possibility that the two isoforms
have some unique functions [49,50].
β
TrCP1-deficient MEFs have markedly reduced
growth rates compared to wild-type cells. This results from aberrant mitosis and
increased apoptosis [50]. The mitotic defect appears to be caused by stabilization
of Emi1, a mitosis inhibitor, which contains a canonical
β
β
TrCP destruction motif
and may be specifically targeted by
β
TrCP1. Nakayama and colleagues noted a
partial block in degradation of I
κ
B
α
and
β
-catenin in splenocytes, thymocytes, and
embryonic fibroblasts of the
TrCP1-deficient mice [49]. In contrast, Guardavaccaro
and colleagues did not detect any reduction in I
β
κ
B or
β
-catenin degradation in
β
TrCP1-deficient mouse embryo fibroblasts (MEFs) [50]. Taken together, these data
may indicate that
β
TrCP 1 and 2 have redundant functions with respect to the
regulation of I
-catenin ubiquitination. Consistent with this interpretation,
there are no obvious physiological consequences of impaired NF-
κ
B
α
and
β
κ
B activation or
aberrant Wnt activity in
TrCP1 knockout mice. There are also no clear defects
suggesting aberrant mitosis, with the exception of reduced fertility reported in one
of the knockout strains, that appears to result from defective spermatogenesis [50].
Assuming that
β
TrCP1 was completely eliminated in both studies, the paucity of
developmental, physiologic, or clear pathologic sequella in the mutant mice suggests
a redundant role for the two
β
B and
Wnt regulation. Furthermore, the elimination of one isoform may be compensated
by upregulation of the other one (Davis and Ben-Neriah, unpublished data), as with
many other genetic backup circuits [51]. It is also possible that Emi1 stability is
coregulated by both
β
TrCP isoforms, at least with respect to NF-
κ
TrCP isoforms
in vivo
, perhaps by formation of a heterodimer
[50]. Further conclusions about the possible functional divergence of
β
β
TrCP 1 and
2 await the generation of
β
TrCP2
-/-
and
β
TrCP1
-/-
β
TrCP2
-/-
mouse strains.
4.4
ACTIVATION OF NF-
κ
B BY STRESS
Genotoxic stresses, such as ultraviolet (UV) light and DNA damage, activate NF-
B
transcription factors to mediate both pro- and antiapoptotic functions [32,52]. Con-
siderable research has been carried out into the mechanisms by which cellular
κ
stresses activate NF-
κ
B.
Chapter 3
briefly discusses some new data, suggesting a
