Biology Reference
In-Depth Information
ERK1/2, JNK1/2/3, p38 MAPK). All MAP kinases are soluble proteins,
but phosphatase activity in the cytoplasm makes it highly likely that an
upstream kinase would be dephosphorylated before it finds its target by ran-
dom diffusion. In addition, there are many different MAPKs, MAPKKs, and
especially MAPKKKs in the cell.
174
Therefore, productive complexes con-
taining matching components are usually assembled by scaffolding proteins
that bring the three kinases together, thereby facilitating signaling.
175
Arrestins were shown to promote the activation of JNK3,
148
ERK1/2,
149
and p38MAPK
168
by scaffolding the relevant three-kinasemodules. The first
reports suggested that receptor activation is necessary for arrestin-dependent
scaffolding of MAPK cascades.
148,149,168
Subsequent studies confirmed
that receptor binding is a prerequisite for arrestin-dependent activation
of ERK1/2.
133,136
However, arrestin-3-mediated facilitation of JNK3
phosphorylation was shown to be receptor-independent,
159,162
indicating
that free arrestin-3 performs this function.
136,158
This was unambiguously
confirmedby reconstitutionof an arrestin-3-MKK4-JNK3 signalingmodule
in vitro
from purified proteins in the absence of the receptor.
160
Regardless of whether free, receptor-bound, or both forms of arrestins
scaffold MAPK cascades, several ways of manipulating MAPK signaling
by mutant arrestins are conceivable. First, targeted mutations disrupting
an interaction with one of the kinases would yield arrestins that can only
form incomplete complexes, thereby sequestering the other two kinases
and suppressing the activation of a particular MAPK. Second, a mutant
that binds all kinases but holds them in a “wrong” configuration not con-
ducive to signal transduction would serve this purpose even better. Third,
mutations enhancing the interactions with one or more of the kinases in
a particular cascade would channel signaling in that direction. Finally, muta-
tions that create preference for ERK1/2 over JNK3 would facilitate
prosurvival and suppress proapoptotic signaling, while mutations biasing
arrestin toward JNK3 would have the opposite effect.
Even though the binding sites of MAP kinases remain unknown or
poorly defined, several signaling-biased forms of arrestins that significantly
affect MAPK signaling were created. The first in this line was arrestin-2-
R307A mutant, which was shown to have impaired c-Raf1 binding, while
interacting with ERK1/2 and MEK1 normally.
157
In contrast to WT
arrestin-2, the expression of this mutant did not rescue
b
2AR-dependent
arrestin-mediated ERK1/2 activation in arrestin-2/3 double knockout
cells.
157
Whether this mutant can suppress ERK1/2 activation via other
scaffolds still remains to be elucidated. Interestingly, a homologous K308A
Search WWH ::
Custom Search