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FIGURE 7.3
Functional evidence of 5-HT 4 R dimerization at the cell surface. Myc-5-HT 4 RWT (in red, A, B,
and C), RhoTag-5-HT 4 R-D 100 A (in green, D, E, F), or both (G-O) were transiently expressed
in HEK-293 cells and visualized by immunofluorescence confocal microscopy. Cells are
fixed in basal conditions (A, D, G, J, and M) or after a 30 min exposure with 10
m
M of either
5-HT (B, E, H, K, and N) or BIMU8 (C, F, I, L, and O).
bind to D 100 A, induced an internalization of this mutant receptor, when the D 100 A
mutant was coexpressed with WT receptor ( Fig. 7.3 K and N). Indeed, 5-HT induced
the internalization of WT/WT dimers ( Fig. 7.3 H and N, red dots) and of WT/D 100 A
dimers ( Fig. 7.3 H, K, and N, yellow dots), whereas D 100 A/D 100 A dimers remained at
the cell surface ( Fig. 7.3 K and N, green dots). All types of complexes internalized
after a 30 min exposure with BIMU8 ( Fig. 7.3 I, L, and O). The simplest explanation
of these results implicates the existence of a WT/D 100 A dimer. When the WT mono-
mer of the WT/D 100 A dimer was occupied by 5-HT, both WT and D 100 A protomers
were internalized.
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