Suitability of GRK antibodies for individual detection and quantification of GRK isoforms in western blots

G protein-coupled receptors (GPCRs) are regulated by GPCR kinases (GRKs) which phosphorylate intracellular domains of the active receptor. This leads to the recruitment of arrestins resulting in desensitization and internalization of the GPCR. Aside from acting on GPCRs, GRKs regulate a variety of membrane, cytosolic, and nuclear proteins not only via phosphorylation but also by acting as scaffold. This multifunctionality is also reflected by their diverse roles in pathological conditions like cancer, influenza infection, malaria, and metabolic disease. Reliable tools to study GRKs are the key to specify their role in complex cellular signaling networks. Thus, we examined the specificity of eight commercially available antibodies targeting the four ubiquitously expressed GRK2, GRK3, GRK5, and GRK6 in western blot analysis. We thereby identified one antibody that did not recognize its antigen, as well as antibodies that showed unspecific signals or cross reactivity. Thus, we strongly recommend testing any antibody with exogenously expressed proteins to clearly confirm identity of the obtained western blot results. Utilizing the most suitable antibodies we established the western blot-based, cost-effective, simple tag-guided analysis of relative protein abundance (STARPA). This method allows comparison of protein levels obtained by immunoblotting with different antibodies. Furthermore, we applied STARPA to determine GRK protein levels in five commonly used cell lines revealing differential isoform expression.