G Protein-coupled Receptor Kinase (GRK)

G protein-coupled receptor kinases (GRKs) are a family of serine/threonine kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. On the basis of their sequence homology, GRKs can be subclassified into three groups: the visual subfamily (GRK1 and GRK7), the GRK4 subfamily (GRK4, GRK5 and GRK6) and the β-AR kinase (βARK) subfamily (GRK2 and GRK3). GRK1 and GRK7 are primarily expressed in retinal photoreceptor cells, whereas GRK4 is most abundantly expressed in the testes. GRK2, GRK3 and GRK5 are ubiquitously expressed, albeit to various extents in different tissues. In the heart, GRK2 and GRK5 are most prominent, whereas GRK3 and GRK6 are expressed at low levels. Functionally, all GRKs mediate the uncoupling and internalization of activated GPCRs. All GRKs are primarily localized to the cytosol and plasma membrane. GRK1, GRK4, GRK5, GRK6 and GRK7 are basally localized to the membrane. Conversely, GRK2 and GRK3 are primarily localized to the cytosol and translocate to the membrane after receptor stimulation.