Rapid regulation of G-protein-coupled receptors (GPCRs) involves agonist-promoted receptor phosphorylation by GPCR kinases (GRKs) [<cite idref="PUB00003012"/>]. This processis followed by arrestin binding and transient receptor internalisation. It has been shown that beta-adrenergic receptor kinase (beta ARK-1 or GRK2)follows a similar pattern of internalisation upon agonist activation ofbeta(2)-adrenergic receptors (beta(2)AR) and that beta ARK expressionlevels modulate receptor sequestration [<cite idref="PUB00003012"/>]. Such studies indicate afunctional relationship between receptor phosphorylation and sequestration,showing that beta ARK not only translocates from the cytoplasm to theplasma membrane in response to receptor occupancy, but also sharesendocytic mechanisms with the beta(2)AR [<cite idref="PUB00003012"/>]. These results suggest a rolefor beta ARK in the sequestration process, or involvement of receptorinternalisation in the intracellular trafficking of the kinase [<cite idref="PUB00003012"/>].