Biology Reference
In-Depth Information
domain for binding of the sole SUMO-E2 enzyme, UBC9. In bovine
b
-arrestin2, SUMOylation predominantly occurs at a specific lysine residue
and is important for
b
-arrestin interaction with the endocytic adaptor pro-
tein
b
-adaptin2.
54
The SUMO-E3 ligase that mediates this modification
remains to be identified.
2.2. Biology: Kinetics and stability
7TMR-stimulated
b
-arrestin2 ubiquitination can be categorized into two
patterns: (1) transient ubiquitination which occurs rapidly (e.g., acute stim-
ulation of the
b
2
AR by the agonist isoproterenol leads to transient
b
-arrestin
ubiquitination signals which are rapidly reduced to baseline levels within a
fewminutes) and (2) sustained ubiquitination in which the signals evoked by
agonist treatment do not deteriorate for up to 1 h or more (e.g., stimulation
of AT
1a
R by angiotensin II leads to sustained
b
-arrestin ubiquitination;
Fig. 7.3
A and B; Refs.
55,56
). Apart from routinely used Western blot tech-
niques mentioned above, both transient and sustained ubiquitination of
b
-arrestin as induced by the
b
2
AR and the V
2
vasopressin receptor (V
2
R)
are detectable as dynamic modifications in live cells by monitoring BRET
(bioluminescence resonance energy transfer) between
b
-arrestin2 fused with
luciferase and GFP
2
-tagged ubiquitin.
57
The two patterns of
b
-arrestin
ubiquitination parallel previously shown trafficking profiles of green fluores-
cent protein (GFP)-tagged
b
-arrestins, in which transient recruitment to
activated
b
2
ARs at the plasma membrane defines “class A” and stable
recruitment to activated AT
1a
Rs, first to the plasma membrane followed
by cointernalization of receptor-
b
-arrestin complexes, defines “class B”
receptor.
58
In general, class A receptors recycle and resensitize at the plasma
membrane more rapidly than the class B receptors, and their affinities for the
two
b
-arrestin isoforms are also different. Class A receptors preferentially
recruit
b
-arrestin2, whereas class B receptors bind both
b
-arrestins with
equal affinity. These internalization, recycling,
b
-arrestin binding, and
ERK activation patterns are attributed to particular serine-threonine-rich
phosphorylation motifs found only in the class B receptors.
58,59
Interest-
ingly, when the C-terminal region of a class A receptor is replaced with that
of a class B receptor, trafficking, ERK activation, and ubiquitination patterns
are transformed to the class B type.
55,58,59
Translational fusion of Ub to
b
-arrestin2 promotes its sustained binding to even a class A receptor,
whereas removal of all ubiquitination sites results in transient recruitment
to even class B receptors (
Fig. 7.3
C and D; Refs.
51,55,56
). These findings
Search WWH ::
Custom Search