MAPK-dependent Degradation of G Protein-coupled Receptor Kinase 2

G protein-coupled receptor kinase 2 (GRK2) is a key modulator of G protein-coupled receptors (GPCR). Altered expression of GRK2 has been described to occur during pathological conditions characterized by impaired GPCR signaling. We have reported recently that GRK2 is rapidly degraded by the proteasome pathway and that β-arrestin function and Src-mediated phosphorylation are involved in targeting GRK2 for proteolysis. In this report, we show that phosphorylation of GRK2 by MAPK also triggers GRK2 turnover by the proteasome pathway. Modulation of MAPK activation alters the degradation of transfected or endogenous GRK2, and a GRK2 mutant that mimics phosphorylation by MAPK shows an enhanced degradation rate, thus indicating a direct effect of MAPK on GRK2 turnover. Interestingly, MAPK-mediated modulation of wild-type GRK2 stability requires β-arrestin function and is facilitated by previous phosphorylation of GRK2 on tyrosine residues by c-Src. Consistent with an important physiological role, interfering with this GRK2 degradation process results in altered GPCR responsiveness. Our data suggest that both c-Src and MAPK-mediated phosphorylation would contribute to modulate GRK2 degradation, and put forward the existence of new feedback mechanisms connecting MAPK cascades and GPCR signaling. G protein-coupled receptor kinase 2 (GRK2) is a key modulator of G protein-coupled receptors (GPCR). Altered expression of GRK2 has been described to occur during pathological conditions characterized by impaired GPCR signaling. We have reported recently that GRK2 is rapidly degraded by the proteasome pathway and that β-arrestin function and Src-mediated phosphorylation are involved in targeting GRK2 for proteolysis. In this report, we show that phosphorylation of GRK2 by MAPK also triggers GRK2 turnover by the proteasome pathway. Modulation of MAPK activation alters the degradation of transfected or endogenous GRK2, and a GRK2 mutant that mimics phosphorylation by MAPK shows an enhanced degradation rate, thus indicating a direct effect of MAPK on GRK2 turnover. Interestingly, MAPK-mediated modulation of wild-type GRK2 stability requires β-arrestin function and is facilitated by previous phosphorylation of GRK2 on tyrosine residues by c-Src. Consistent with an important physiological role, interfering with this GRK2 degradation process results in altered GPCR responsiveness. Our data suggest that both c-Src and MAPK-mediated phosphorylation would contribute to modulate GRK2 degradation, and put forward the existence of new feedback mechanisms connecting MAPK cascades and GPCR signaling. G protein-coupled receptors (GPCR) 1The abbreviations used are: GPCR, G protein-coupled receptors; β2AR, β2-adrenergic receptor; ERK, extracellular signal-regulated kinase; LPA, lysophosphatidic acid; MAPK, mitogen-activated protein kinase; MEK, MAPK kinase; PKC, protein kinase C; GRKs, G protein-coupled receptor kinases. detect a broad spectrum of extracellular signals at the plasma membrane, thereby modulating key cellular functions as diverse as growth, differentiation, inflammation, or neurotransmission (1Luttrell L.M. Daaka Y. Lefkowitz R.J. Curr. Opin. Cell Biol. 1999; 11: 177-183Crossref PubMed Scopus (612) Google Scholar, 2Pierce K.L. Premont R.T. Lefkowitz R.J. Nat. Rev. Mol. Cell. Biol. 2002; 3: 639-650Crossref PubMed Scopus (2125) Google Scholar). Agonist-occupied receptors promote the activation and dissociation of heterotrimeric G proteins into α and βγ subunits, both of which regulate a wide variety of effector systems. In addition, agonist stimulation also leads to the deactivation of GPCR signaling (desensitization) by triggering receptor phosphorylation by specific G protein-coupled receptor kinases (GRKs) and binding of the cytosolic proteins β-arrestins to the phosphorylated receptor (3Carman C.V. Som T. Kim C.M. Benovic J.L. J. Biol. Chem. 1998; 273: 20308-20316Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar, 4Pitcher J.A. Freedman N.J. Lefkowitz R.J. Annu. Rev. Biochem. 1998; 67: 653-692Crossref PubMed Scopus (1072) Google Scholar). GRK2 is a ubiquitous member of the GRK family, which has been shown to modulate a variety of GPCRs (5Aragay A.M. Ruiz-Gomez A. Penela P. Sarnago S. Elorza A. Jimenez-Sainz M.C. Mayor Jr., F. FEBS Lett. 1998; 430: 37-40Crossref PubMed Scopus (64) Google Scholar, 6Lombardi M.S. Kavelaars A. Heijnen C.J. Crit. Rev. Immunol. 2002; 22: 141-163Crossref PubMed Google Scholar). The same regulatory molecules that contribute to receptor uncoupling from G proteins also regulate GPCR endocytosis, intracellular trafficking, and resensitization and participate in the modulation of mitogen-activated protein kinase (MAPK) cascades by GPCR (7Ferguson S.S. Pharmacol. Rev. 2001; 53: 1-24PubMed Google Scholar). Thus, β-arrestins mediate the recruitment of clathrin and β2-adaptin to allow for receptor internalization and also act as scaffold molecules by bridging receptors with signaling proteins such as c-Src, thus facilitating the activation of the ERK/MAPK cascade by GPCR (8Luttrell L.M. Ferguson S.S. Daaka Y. Miller W.E. Maudsley S. Della Rocca G.J. Lin F. Kawakatsu H. Owada K. Luttrell D.K. Caron M.G. Lefkowitz R.J. Science. 1999; 283: 655-661Crossref PubMed Scopus (1264) Google Scholar, 9McDonald P.H. Lefkowitz R.J. Cell. Signal. 2001; 13: 683-689Crossref PubMed Scopus (113) Google Scholar). In addition, the isoforms of β-arrestins can directly interact with components of two different MAPK cascades bringing these molecules into close proximity with the receptor complex (10Luttrell L.M. Roudabush F.L. Choy E.W. Miller W.E. Field M.E. Pierce K.L. Lefkowitz R.J. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 2449-2454Crossref PubMed Scopus (704) Google Scholar, 11McDonald P.H. Chow C.W. Miller W.E. Laporte S.A. Field M.E. Lin F.T. Davis R.J. Lefkowitz R.J. Science. 2000; 290: 1574-1577Crossref PubMed Google Scholar). On the other hand, GRK2 has been shown to interact with the ARF modulator GIT, with phosphatidylinositol 3′-OH kinase γ, and the G protein αq subunit (12Pao, C. S., and Benovic, J. L. (2002) Science's STKE http://www.stke.org/cgi/content/full/OC_sigtrans;2002/153/pe42, and references thereinGoogle Scholar) and to phosphorylate non-receptor substrates such as tubulin, synucleins, or phosducin (13Ruiz-Gomez A. Humrich J. Murga C. Quitterer U. Lohse M.J. Mayor Jr., F. J. Biol. Chem. 2000; 275: 29724-29730Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar), thus also extending the cellular functions of GRK2 beyond GPCR desensitization. The key role that GRKs and β-arrestins play in GPCR signaling and modulation suggests that the overall activity of GPCRs would be strongly dependent on the cellular complement and functionality of these proteins. Consistent with this idea, the altered expression of GRK2 described in several pathological conditions such as hypertension (14Gros R. Benovic J.L. Tan C.M. Feldman R.D. J. Clin. Invest. 1997; 99: 2087-2093Crossref PubMed Scopus (200) Google Scholar), congestive heart failure (15Ungerer M. Bohm M. Elce J.S. Erdmann E. Lohse M.J. Circulation. 1993; 87: 454-463Crossref PubMed Scopus (763) Google Scholar), or rheumatoid arthritis (6Lombardi M.S. Kavelaars A. Heijnen C.J. Crit. Rev. Immunol. 2002; 22: 141-163Crossref PubMed Google Scholar) has been correlated with impaired GPCR signaling in these situations. GRK2 activity and subcellular distribution are tightly regulated by interactions with Gβγ subunits, lipids, agonist-activated proteins and or phosphorylation by other kinases in Benovic J.L. 2000; PubMed Scopus Google Scholar, Lefkowitz R.J. Mol. Pharmacol. PubMed Scopus Google Scholar, P. C. Mayor Jr., Cell. Signal. Google Scholar). On the other hand, the mechanisms that GRK2 cellular and that in physiological and pathological conditions have to be In this we have reported recently P. Ruiz-Gomez A. Mayor Jr., F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) that GRK2 is degraded by the proteasome pathway and that kinase turnover enhanced GPCR we have shown that β-arrestin function and c-Src activity are involved in GRK2 proteolysis. binding of β-arrestin to GPCRs for the recruitment of c-Src, to phosphorylation of GRK2 on tyrosine residues and targeting for degradation P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). the of that GRK2 stability also by of data P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). We and A. Sarnago S. Mayor Jr., F. Mol. Pharmacol. 2000; PubMed Scopus Google Scholar, J.A. J.L. Lefkowitz R.J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) have described that GPCR activation the of MAPK and GRK2 in the same complex and that MAPK In this report, we show that MAPK-mediated GRK2 phosphorylation triggers GRK2 degradation in a process that to be facilitated GRK2 is Our data that both c-Src and MAPK would contribute to modulate GRK2 protein stability by GRK2 degradation, and put forward new regulatory MAPK cascades and GPCR signaling. and and from the and from and from and from of mutant and the of and by J. The of from M. The of other expression and reported P. Ruiz-Gomez A. Mayor Jr., F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar, P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). other of the of GRK2 at to by a Thus, this mutant by the and of the in with the mutant and The into in The mutant with the the and the and by of the the specific at with and in the mutant as described P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). Cell and and in with at in a in with transfected by the as described P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). as to the of transfected of wild-type and mutant proteins by with specific to expression Cell and and as described P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). In β2AR, receptor activation by during in with and receptor activation with the a from and during In transfected or endogenous to for by and with for and with for The kinase or and the c-Src tyrosine kinase to or In endogenous receptors in in for with the for The proteasome and and during the and and in and cellular with the specific GRK2 as reported P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). in and to or to to be with an and with In the of GRK2 phosphorylated on in the by an specific from MAPK activation in cellular by a MAPK The same with a MAPK to MAPK activation to MAPK protein to protein of the different wild-type and mutant as described P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). expression by a a by on previous A. Sarnago S. Mayor Jr., F. Mol. Pharmacol. 2000; PubMed Scopus Google Scholar, J.A. J.L. Lefkowitz R.J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) that GRK2 and MAPK as as GRK2 phosphorylation by the we modulation of the ERK/MAPK pathway have effect on GRK2 this and GRK2 turnover by in conditions that would MAPK shows that the enhanced GRK2 degradation by agonist strongly in the of the or of a data that MAPK activation is involved in GRK2 proteolysis. Interestingly, of MAPK activation also leads to a GRK2 turnover that in the of β2-adrenergic receptor different endogenous GPCRs the activity of P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) are modulating GRK2 stability a also MAPK Consistent with a role for activity in GRK2 the degradation of GRK2 in with the of GRK2 of with data the of MAPK in GRK2 degradation, we the expression of of the MAPK cascade would GRK2 turnover. transfected with GRK2 and a mutant of an regulatory M. J.L. P. T. Mol. Cell. Biol. PubMed Scopus Google Scholar), thus a MAPK activation MAPK protein The expression of this mutant GRK2 degradation as by of kinase with in the expression of an mutant that MAPK activity in a C.J. Mol. Cell. Biol. 1998; PubMed Scopus Google Scholar) also the GRK2 degradation of kinase of of GRK2 also by a mutant The of GRK2 by in by of a mutant thereby that the effect these results to a role for MAPK stimulation in GRK2 turnover. In to the of this regulatory we the role of MAPK activity in the degradation of endogenous GRK2 in We have reported P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) that GRK2 rapidly degraded by the proteasome pathway in these and that this process enhanced by activation of the endogenous receptor and by tyrosine kinase shown in the of the which strongly MAPK activation endogenous GRK2 degradation as by GRK2 by are in in these conditions is the of other cellular proteins The effect of MAPK on GRK2 degradation be a of the direct phosphorylation of GRK2 by MAPK or an effect by MAPK-mediated phosphorylation of other proteins. In this has been reported that GRK2 is phosphorylated in at the MAPK phosphorylation J.A. J.L. Lefkowitz R.J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). the role of direct MAPK phosphorylation of GRK2 on degradation, we at to both MAPK phosphorylation or to shown in the turnover of the mutant in as with wild-type GRK2, and degradation to that for GRK2 receptor stimulation P. Ruiz-Gomez A. Mayor Jr., F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) or in the of of the MAPK pathway of protein of of wild-type protein in the of of by specific of the proteasome pathway which also degradation of wild-type GRK2 GPCR activation P. Ruiz-Gomez A. Mayor Jr., F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). data are with the that receptor leads to MAPK activation and that direct phosphorylation of GRK2 by MAPK triggers degradation of the mutant with that of wild-type GRK2 and by the proteasome specific the in wild-type GRK2 degradation that in the of is with the mutant results that GRK2 phosphorylation by MAPK a role in GRK2 degradation, such is for targeting of the to the of and to GRK2 We in the mechanisms turnover. GPCR can c-Src and MAPK, and GRK2 can be phosphorylated by both kinases in an we have recently shown P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) that phosphorylation is involved in GRK2 we the role of tyrosine phosphorylation on Thus, tyrosine residues for c-Src phosphorylation P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) to in the Interestingly, the turnover of this mutant impaired as with or wild-type GRK2 of protein of and is to the described for the mutant of protein at of data that the modulation of GRK2 degradation by is the a tyrosine the of different regulatory in GRK2 Interestingly, of the residues for c-Src phosphorylation in the mutant has effect on turnover with a mutant of the kinase of c-Src which wild-type GRK2 degradation P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar), the of degradation of the mutant results would suggest that GRK2 is phosphorylated by MAPK, degradation of these data suggest that c-Src and MAPK-mediated phosphorylation GRK2 to the proteasome pathway GRK2 is phosphorylated by MAPK, for degradation, and tyrosine phosphorylation is GRK2 be phosphorylated by MAPK, a tyrosine pathway. In both wild-type GRK2 and mutant are stimulation of to occur these two turnover shows that the expression of the degradation of the GRK2 mutant the of the the GRK2 turnover. data suggest for wild-type GRK2, MAPK phosphorylation be modulating the stability of the of the GRK2 cellular phosphorylated on tyrosine of stimulation of endogenous receptors in are with this of receptors leads to both and MAPK activation of the is dependent on tyrosine kinase activity We that stimulation the phosphorylation of endogenous GRK2 on as with a specific this phosphorylated Interestingly, the phosphorylation of this MAPK is in the of the c-Src tyrosine kinase MAPK activation is these is to that a cellular is for of GRK2 stability by In this we have reported P. Ruiz-Gomez A. Mayor Jr., F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar) that the is degraded in is to to promote the recruitment of β-arrestin and molecules such as c-Src to the of the β-arrestin can turnover P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). Interestingly, of promote the that are for GRK2 turnover. we of with and effect on the degradation of this The of a in the turnover of by β-arrestin In other with and wild-type GRK2 in the of different of wild-type effect on the enhanced GRK2 degradation by indicating that endogenous β-arrestin in these are to GRK2 degradation as P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). On the expression of is to c-Src, P. C. Mayor Jr., Cell. Signal. Google Scholar) the degradation by to has the same effect as wild-type results that MAPK-mediated of GRK2 turnover requires phosphorylation of GRK2 on tyrosine residues by c-Src and the of β-arrestin as a protein is in with the of β-arrestin to c-Src P. C. Mayor Jr., Cell. Signal. Google Scholar) and to signaling and (10Luttrell L.M. Roudabush F.L. Choy E.W. Miller W.E. Field M.E. Pierce K.L. Lefkowitz R.J. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 2449-2454Crossref PubMed Scopus (704) Google Scholar), and proteins to promote GRK2 degradation The complex mechanisms of of GRK2 stability suggest that such modulation have a role in the of GPCR signaling. we in GRK2 and turnover cellular to GPCR In a of we that agonist of endogenous in endogenous GRK2 of cellular GRK2 complement as with with Interestingly, MAPK activation in to enhanced in with GRK2 as a of activation conditions in activation in data from with a in the of GRK2 in an endogenous to to other results that degradation of GRK2 contribute to GPCR in physiological conditions characterized by GPCR The of an of GRK2 turnover in GPCR signaling is also by in and wild-type GRK2 or the mutant in the endogenous wild-type GRK2 protein stimulation of kinase as with in protein expression in to with activation of endogenous receptors MAPK activation impaired as with wild-type conditions in the In these we in wild-type the in MAPK activation in to as of that is in an endogenous the in MAPK wild-type GRK2 and is to stability components of the MAPK cascade are as MAPK activation in wild-type or mutant GRK2 these strongly suggest that in GRK2 turnover are important for the of signaling. In this we show that MAPK phosphorylation the degradation of GRK2, thus forward a new MAPK cascades and GPCR signaling. of the that MAPK activation GRK2 turnover. In transfected direct stimulation of the ERK/MAPK pathway at different GRK2 proteolysis. On the the expression of or the of of the cascade both and GRK2 the of such the turnover of endogenous GRK2 in and results in GRK2 cellular thus indicating that MAPK-mediated modulation of GRK2 degradation has in endogenous systems. the that a mutant that would the phosphorylation by MAPK J.A. J.L. Lefkowitz R.J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), shows an enhanced degradation with wild-type GRK2 a direct effect of MAPK-mediated GRK2 phosphorylation on the modulation of GRK2 On the other hand, a mutant shows degradation in to GPCR with a role for MAPK in GRK2 proteolysis. would also previous of that the mutant GRK2 degradation in a receptor P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). The effect of the mutant would be by reported effect on the activation of MAPK cascades J.L. M. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). The of GRK2 stability as a complex process different signaling We have reported recently P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) that recruitment of β-arrestin and Src-mediated GRK2 phosphorylation on tyrosine residues are signals that GRK2 degradation the proteasome pathway. key is and mechanisms contribute to GRK2 turnover. In to this we have different conditions the turnover of a variety of GRK2 proteins residues for MAPK or c-Src a mutant that mimics phosphorylation by MAPK or of GRK2 is the turnover of the GRK2 mutant is with to the wild-type the degradation is would suggest that MAPK-mediated modulation is for signals can GRK2 proteolysis. The pathway for degradation to be dependent on tyrosine the turnover of the mutant is On the phosphorylation by MAPK is by the the protein rapidly to degradation, activity is or tyrosine phosphorylation of the mutant is by the tyrosine residues these data suggest that tyrosine or phosphorylation can GRK2 degradation by the proteasome pathway. results also the of a role of GRK2 tyrosine phosphorylation for the of wild-type GRK2 turnover. The degradation of a GRK2 mutant to be phosphorylated by such as is enhanced by or of MAPK be that GRK2 is a for In has been reported that GRK2 of to be phosphorylated by MAPK J.A. J.L. Lefkowitz R.J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). In with we that phosphorylation of endogenous GRK2 in is in the of a c-Src or in addition, GRK2 be in an cellular or complex that the of GRK2 with MAPK and other molecules involved in the degradation In this we that stimulation of the MAPK cascade by is to the β-arrestin is in P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) thus to the of β-arrestin function also in modulation of GRK2 the degradation of wild-type GRK2 stimulation of MAPK cascades is in the of a mutant that is to c-Src to the receptor data the of an cellular for MAPK-mediated of GRK2 is that β-arrestin function to play a role in both P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar) and GRK2 degradation In to c-Src (8Luttrell L.M. Ferguson S.S. Daaka Y. Miller W.E. Maudsley S. Della Rocca G.J. Lin F. Kawakatsu H. Owada K. Luttrell D.K. Caron M.G. Lefkowitz R.J. Science. 1999; 283: 655-661Crossref PubMed Scopus (1264) Google Scholar) and thus for GRK2 phosphorylation on tyrosine residues P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar, S. Elorza A. Mayor Jr., F. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), β-arrestin has been reported to key components of the MAPK cascade MEK, and in the of GPCR P.H. Lefkowitz R.J. Cell. Signal. 2001; 13: 683-689Crossref PubMed Scopus (113) Google Scholar, L.M. Roudabush F.L. Choy E.W. Miller W.E. Field M.E. Pierce K.L. Lefkowitz R.J. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 2449-2454Crossref PubMed Scopus (704) Google Scholar). The scaffold function of β-arrestin would contribute to the and of the the MAPK, and GRK2 β-arrestin has been reported recently P.H. Lefkowitz R.J. Science. 2001; PubMed Scopus Google Scholar) to interact with and other as involved in and β-arrestin these or other β-arrestin also contribute to GRK2 to the degradation The in for the mechanisms of GRK2 degradation the proteasome pathway. activation would promote the recruitment of GRK2, and c-Src to the GPCR in tyrosine phosphorylation of GRK2 S. Elorza A. Mayor Jr., F. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). process can be at several to impaired GRK2 would to the receptor promote the binding of β-arrestin and c-Src can be by β-arrestin phosphorylation of GRK2 on tyrosine residues can be by with β-arrestin (8Luttrell L.M. Ferguson S.S. Daaka Y. Miller W.E. Maudsley S. Della Rocca G.J. Lin F. Kawakatsu H. Owada K. Luttrell D.K. Caron M.G. Lefkowitz R.J. Science. 1999; 283: 655-661Crossref PubMed Scopus (1264) Google Scholar) or the W.E. Maudsley S. S. Luttrell L.M. Lefkowitz R.J. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar), by tyrosine kinase shown in the or GRK2 residues for phosphorylation P. Elorza A. Sarnago S. Mayor Jr., F. J. 2001; PubMed Scopus (111) Google Scholar). phosphorylation of GRK2 would be for triggering kinase the proteasome pathway. 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