Phosphorylation of GPCR Kinase 2 in Intact Cells: A Proteomic Approach

G protein‐coupled receptor (GPCR) kinases (GRKs) were first discovered by their ability to phosphorylate GPCRs in an agonist‐dependent manner. GRKs belong to the AGC kinase family of kinases. Activation of AGC kinases is often accompanied by phosphorylation of activation loop residues as well as other conformational changes in catalytically important residues. Crystal structures of GRK2 suggest that the activation segment region is in a relatively “active” conformation in the absence of phosphorylation. Nonetheless, a phosphoproteomic study (Oppermann et al., 2009) indicated that GRK2 Ser350 and Thr353 can be phosphorylated in intact cancer cells. Ser350 and Thr353 reside in the activation loop and P+1 site of the peptide‐binding site, respectively. To test the role of Ser350 and Thr353 phosphorylation in GRK2 function, we substituted alanine and aspartate at these two positions and tested the ability to phosphorylate the beta‐adrenergic receptor. We found that Thr353 is particularly important for phosphorylation of beta‐adrenergic receptor by GRK2 in COS‐7 cells, since conversion to Ala or Asp prevented substrate phosphorylation. In cells transfected with both beta‐adrenergic receptor and GRK2, multiple forms of GRK2 were detected in cells treated with agonist. The multiple forms are consistent with various phosphorylation states. Our goal is to assess Ser350/Thr353 phosphorylation in intact cells using phosphoproteomic approaches. To this end, we engineered a hexahistidine tag on GRK2 expressed from a mammalian cell vector to facilitate its purification and have demonstrated that the tagged kinase is fully functional in agonist‐dependent receptor phosphorylation. We are further developing methods to assess GRK2 phosphorylation by isoelectric focusing and mass spectrometry. Support or Funding Information This work was funded by the National Science Foundation MCB0744739 and Siena College Center for Undergraduate Research and Creative Activity (CURCA). This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .