Examination of ubiquitination and SUMOlyation of GRK4γ (1055.3)

Abstract

G protein‐coupled receptor kinases 4γ (GRK4γ) is associated with hypertension. However, the molecular mechanism of action of GRK4γ remains largely unknown. From a proteomic examination of the binding partners of GRK4γ, we identified Gαs, Heat Shock Protein 90 (Hsp90) and an E3 ubiquitin ligase as potential binding partners. Additionally, previous studies suggested that GRK4γ is ubiquitinated. GRK2 and GRK5 are known to be ubiquitinated while SUMOylation, a small Ub‐like modifier protein that also attaches to lysine, has not been extensively studied in any GRKs. Therefore, we tested the hypothesis that GRK4γ can be ubiquitinated and SUMOylated. To demonstrate Ubiquitination and SUMOylation, we immunoprecipitated (IPed) GRK4γ and blotted for Ub or SUMO. The data clearly showed that GRK4γ is ubiquitinated but not SUMOylated. Interesting, GRK4γ ubiquitination is significantly enhanced after stimulation of MG132, a cell‐permeable proteasome inhibitor, but remains unchanged in the presence of two other proteasome inhibitors (Lactacystin and proteasome inhibitor I) which is counter to the previous studies. Further studies will be conducted to identify the role of GRK4γ activity in relation to GRK4γ ubiquination as well as identify the ubiquitination sites on GRK4γ and the role that ubiquitination plays in GRK4γ activity.