Biomedical Engineering Reference
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b
2
-adrenergic receptor dimerization and subsequent trafficking to the PM upon
disrupting the putative dimerization motif GXXXGXXXL (Salahpour et al.
2004
) .
In another study, Kobayashi et al. (
2009
) mutated Val179 and Trp183 (two evo-
lutionarily important amino acids in TM 4) of the b
1
-adrenergic receptor. This
resulted in increased BRET
50
values in saturation BRET studies, indicating a reduced
tendency to dimerize, either directly by interfering with the dimerization interface
or indirectly by causing local misfolding that resulted in reduced self-assembly
(Kobayashi et al.
2009
) .
3.8
How Biogenesis of GPCR Oligomers can be In fl uenced
GPCRs are translated and folded in the ER. Often, these GPCRs will form homo- or
heteromers depending on the amount of available GPCRs, ligands, and chaperones.
The equilibrium between the assemblies of a particular GPCR in a homomeric
quaternary structure can be shifted to a heteromeric structure. This can cause a
change in the functionality of the receptor and we can hypothesize that this can
result in a certain physiological effect
in vivo
.
3.8.1
Role of the Availability of Different GPCRs
We have shown for the dopamine D
4
receptor that oligomerization occurs in the ER
and that it plays a role in biogenesis and cell surface targeting of the receptor. BRET
saturation assays suggest that the polymorphic repeat variants of the dopamine D
4
receptor (D
4.2
, D
4.4
and D
4.7
) can form homo- and heteromers, although with a differ-
ent degree of ligand affinity. Namely, BRET
50
values are lower for the homomers
compared to the heteromers and the dopamine D
4.7
receptor is the least capable
of forming heteromers (Van Craenenbroeck et al.
2011
) . Also, when studying
oligomerization between the dopamine D
2
receptor and the different polymorphic
variants of the dopamine D
4
receptor, a similar variability in oligomerization
was detected. Speci fi cally, the BRET
50
value of the dopamine D
4.7
receptor, and the
dopamine D
2
receptor is higher in comparison with the BRET
50
value of the other
heterodimers (Borroto-Escuela et al.
2011
) . Next, BRET
50
values indicate that there
is no significant difference to form homodimers (dopamine D
2
-D
2
receptors) com-
pared to the formation of heterodimers (dopamine D
2
- D
4.2/
/D
4.4
receptor).
However, with a comparable approach, it was shown that the b
1
- and b
2
-adrenergic
receptor have a similar tendency to form homo- and heteromers (Mercier et al.
2002
) .
It is pertinent to remember that all these studies are performed
in vitro
, in cell
lines overexpressing the receptors of interest. We can only speculate what is hap-
pening
in vivo
, for example, how available are alternative GPCRs for assembly
taken into account the timing of gene expression and the spatial aspects of protein
biosynthesis? Considering the hypothesis about the cooperative folding of GPCR
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