Biology Reference
In-Depth Information
leads to reversal of angiotensin II-stimulated
b
-arrestin ubiquitination from
a sustained to a transient pattern, with a corresponding reversal of AT
1a
R-
b
-
arrestin binding from stable endosome-localized complexes to transiently
associated complexes seen only at the plasma membrane. Quite unexpectedly,
however, the same
b
-arrestin mutant is stably ubiquitinated and associated
tightly with two other activated class B receptors, the V
2
R and neurokinin 1
receptor. Furthermore, V
2
R-stimulated
b
-arrestin ubiquitination is impaired
in a quintuple lysine mutant
b
-arrestin2 in which five lysines at positions 18,
107, 108, 207, and 296 are altered to ariginines.
56
These provocative findings
suggest that different 7TMRs are capable of inducing sustained
b
-arrestin
ubiquitination at distinct sites on
b
-arrestin2. Whether the susceptibility of
target lysines in
b
-arrestin is a result of distinct conformational changes or
whether it reflects the activity of distinct E3 ubiquitin ligases that bind and
modify different domains in
b
-arrestin remains an important topic for future
studies.
Unlike stable ubiquitination that is targeted to specific lysines in
b
-arrestin2, transient ubiquitination appears to be a random modification
because it is eliminated only when all the lysines in
b
-arrestin2 are mutated
to arginines (
b
-arrestin2-0K).
51
Although
b
-arrestin2-0K is capable of
equivalent protein-protein interactions with the
b
2
AR as the non-
ubiquitinated wild-type protein
in vitro
, in a cellular context it shows impair-
ment in binding because unlike the wild type, it cannot be ubiquitinated at
proper site(s). In fact, the recruitment of
b
-arrestin2-0K-GFP to either
b
2
AR or V
2
R is very transient and detected only when GRKs are over-
expressed to augment receptor phosphorylation or when activated receptor
complexes are forcibly retained at the plasma membrane by blocking inter-
nalization.
51
Accordingly, ubiquitination of
b
-arrestin2 is not required for its
initial recruitment to the cell-surface 7TMR, but it is required for
prolonging and stabilizing the
b
-arrestin-receptor interaction.
2.3. Functional effects
Apart from stabilizing receptor interaction as discussed above, ubiquitination
of
b
-arrestin has been shown to promote both its endocytic and scaffolding
functions. While earlier studies had indicated a dependence of
b
2
AR inter-
nalization on
b
-arrestin ubiquitination status, later studies carried out with
two modified
b
-arrestins, namely,
b
-arrestin2-0K that lacks ubiquitin
acceptor sites and
b
-arrestin2-Ub which remains stably ubiquitinated have
provided important
insights.
51
b
-Arrestin2-0K does not promote
Search WWH ::
Custom Search