Figure 7. GRK2 and GRK3 are responsible for agonist-induced phosphorylation of the human sst₃ receptor.

A, HEK293 cells stably expressing the sst₃ receptor were transfected with siRNA targeted to GRK2, GRK3, GRK5, and GRK6 or nonsilencing siRNA control (SCR) for 72 hours and then exposed to 1μM SS-14 for 10 minutes (room temperature). Cells were lysed and immunoblotted with anti-pS337/pT341 antibodies. Blots were stripped and reprobed with the phosphorylation-independent anti-HA antibody to confirm equal loading of the gels (anti-HA). Receptor monomers were quantified and expressed as percentage of maximal phosphorylation in SCR-transfected cells. Data correspond to the mean ± SEM from 3 independent experiments. Results were analyzed by two-way ANOVA followed by the Bonferroni post hoc test (*, P < .05). The arrow indicates the position of sst₃ monomers that were used for quantification. B and C, siRNA knockdown of GRK2, GRK3, GRK5, and GRK6 was confirmed by Western blotting. The positions of molecular mass markers are indicated on the left (in kDa).