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The metabotropic glutamate 1 (mGlu 1 ) receptor in cerebellar Purkinje cells plays a key role in motor learning and motor coordination. Here we show that the G proteincoupled receptor kinases (GRK) 2 and 4, which are expressed in these cells, regulate the mGlu 1 receptor by at least in part different mechanisms. Using kinase-dead mutants in HEK293 cells, we found that GRK4, but not GRK2, needs the intact kinase activity to desensitize the mGlu 1 receptor, whereas GRK2, but not GRK4, can interact with and regulate directly the activated G␣ q . In cells transfected with GRK4 and exposed to agonist, ␤-arrestin was first recruited to plasma membranes, where it was co-localized with the mGlu 1 receptor, and then internalized in vesicles. The receptor was also internalized but in different vesicles. The expression of ␤-arrestin V53D dominant negative mutant, which did not affect the mGlu 1 receptor internalization, reduced by 70 -80% the stimulation of mitogen-activated protein (MAP) kinase activation by the mGlu 1 receptor. The agonist-stimulated differential sorting of the mGlu 1 receptor and ␤-arrestin as well as the activation of MAP kinases by mGlu 1 agonist was confirmed in cultured cerebellar Purkinje cells. A major involvement of GRK4 and of ␤-arrestin in agonist-dependent receptor internalization and MAP kinase activation, respectively, was documented in cerebellar Purkinje cells using an antisense treatment to knock down GRK4 and expressing ␤-arrestin V53D dominant negative mutant by an adenovirus vector. We conclude that GRK2 and GRK4 regulate the mGlu 1 receptor by different mechanisms and that ␤-arrestin is directly involved in glutamate-stimulated MAP kinase activation by acting as a signaling molecule.Metabotropic glutamate (mGlu) 1 receptors, which are activated by the excitatory amino acid glutamate, are part of an original family of G protein-coupled receptors (GPCR) called the family 3 GPCRs (1-3). These include all the mGlu receptor subtypes, Ca 2ϩ -sensing and GABA B receptors, and some putative olfactory, pheromone, and taste receptors. Eight subtypes of mGlu receptors have been identified, which are implicated in different aspects of physiology and pathology of the central nervous system. Group I mGlu receptors (mGlu 1 and mGlu 5 ), which stimulate polyphosphoinositide hydrolysis by coupling to G q , are localized in the peripheral parts of postsynaptic dendrites and contribute to the regulation of synaptic plasticity. For example, mGlu 1 receptor present in cerebellar Purkinje cells plays a key role in motor learning and motor coordination. Similar to many other GPCRs, the signal transduction of the mGlu 1 receptor is strictly regulated by multiple mechanisms acting at different levels of signal propagation (1). After prolonged or repeated stimulation, receptors are profoundly desensitized. Protein kinase C is clearly involved in this process, although a protein kinase C-independent component of mGlu 1 receptor desensitization was also observed (4). The activated ␣ subunit of the G q (G␣ q ) ca...

SUMMARY:Proline-rich kinase 2 (Pyk2), also known as CAK␤ (cell adhesion kinase ␤), is a cytoplasmic tyrosine kinase that is structurally related to focal adhesion kinase. Pyk2 is expressed in different cell types including brain cells, fibroblasts, platelets, and other hemopoietic cells. Pyk2 is rapidly tyrosine phosphorylated in response to diverse extracellular signals acting via different post receptor pathways. We have investigated whether this protein kinase is functionally expressed in normal and neoplastic prostate tissues. In this study, we demonstrate that Pyk2 is expressed only in normal epithelial prostate tissue and in benign prostatic hyperplasia, whereas its expression progressively declines with an increasing grade of malignancy of prostate cancer. (Lab Invest 2001, 81:51-59).