Ein stability or increases receptor internalization and down-regulation, as has been described for the Arg3.50(126)Asn CCR5 mutant [52]. This suggests that the role of the DRY motif in activation of CCR5 does not comply with the consensus view [51,53]. The Glu6.30 residue in intracellular loop 3 forms part of the ionic lock in rhodopsin, but many GPCRs, including CCR5, have basic residues in position 6.30 [53]. Crystal structures of the inAH252723 web active CXCR4 chemokine receptor show no interaction between Arg3.50 and Arg6.30 [10,50]. The naturally-occurring Arg6.32(225)Gln CCR5 mutant is partially constitutively active and we hypothesized that Arg6.32(225), which is two residues away from Arg6.30(223), might form alternative interactions that stabilize the inactive CCR5 conformation. Other mutations of Arg6.32(225) did not increase constitutive activity. Decreased expression of these mutants is consistent with the role of basic amino acids in stabilizing membrane-spanning helices [54] although the naturally-occurring Arg6.32(225)Gln mutation did not decrease receptor expression [22]. Furthermore, combining the Thr2.56(82)Lys and Thr2.56(82)Pro mutations with the Arg6.32(225)Gln mutation increased expression of constitutively active mutant CCR5 receptors. The Arg6.32(225)Gln mutation may stabilize a receptorconformation that is less susceptible to internalization or to degradation. The Arg6.32(225)Gln double mutation enhanced expression of constitutively active receptors more effectively in HEK 293 cells than in HOS-CD4-Luc cells. This may result from different receptor trafficking in the two cell lines or it may reflect the generally lower transfection efficiency and receptor expression in HOS-CD4-Luc cells. A proposal that the TxP motif acts as a switch that activates CCR5 was supported by mutations that uncoupled the CCR5 receptor from cellular signaling [20,55] or increased constitutive cellular signaling [21]. The Thr2.56(82)Lys and Thr2.56(82)Pro CCR5 mutants that we tested displayed increased basal IP production and could not be further stimulated by MIP-1b. The same mutants were constitutively active and showed 1655472 no further response to chemokine treatment in a yeast reporter system [21], suggesting that they are fully stabilized in activated conformations. They also constitutively stimulated GTPcS binding in Finafloxacin biological activity stably transfected CHO cells. However, agonist treatment enhanced GTPcS binding [21], suggesting that the Thr2.56(82)Lys and Thr2.56(82)Pro mutations do not fully stabilize the CCR5 conformation that activates the cognate Gai protein. The double mutants, Thr2.56(82)Lys/Arg6.32(225)Gln and Thr2.56(82)Pro/ Arg6.32(225)Gln, both showed basal IP production that was similar to the maximum MIP-1b-stimulated IP production mediated by wild type CCR5, suggesting that they are fully stabilized in activated conformations. However, it is not known whether the CCR5 conformations that activate native Gai signaling pathways are fully stabilized in the double mutant receptors. Mutant receptors with Lys substituted for Thr2.56(82) showed decreased cell surface protein, which may result from decreased receptor stability or stabilization of receptor conformations that constitutively expose cytosolic Ser residues to G protein-coupled receptor kinases, leading to constitutive internalization [44,45,46,47,56]. In contrast, the Thr2.56(82)Pro mutation may stabilize receptor conformations that are not recognized by receptor kinases or are less flexible. The d.Ein stability or increases receptor internalization and down-regulation, as has been described for the Arg3.50(126)Asn CCR5 mutant [52]. This suggests that the role of the DRY motif in activation of CCR5 does not comply with the consensus view [51,53]. The Glu6.30 residue in intracellular loop 3 forms part of the ionic lock in rhodopsin, but many GPCRs, including CCR5, have basic residues in position 6.30 [53]. Crystal structures of the inactive CXCR4 chemokine receptor show no interaction between Arg3.50 and Arg6.30 [10,50]. The naturally-occurring Arg6.32(225)Gln CCR5 mutant is partially constitutively active and we hypothesized that Arg6.32(225), which is two residues away from Arg6.30(223), might form alternative interactions that stabilize the inactive CCR5 conformation. Other mutations of Arg6.32(225) did not increase constitutive activity. Decreased expression of these mutants is consistent with the role of basic amino acids in stabilizing membrane-spanning helices [54] although the naturally-occurring Arg6.32(225)Gln mutation did not decrease receptor expression [22]. Furthermore, combining the Thr2.56(82)Lys and Thr2.56(82)Pro mutations with the Arg6.32(225)Gln mutation increased expression of constitutively active mutant CCR5 receptors. The Arg6.32(225)Gln mutation may stabilize a receptorconformation that is less susceptible to internalization or to degradation. The Arg6.32(225)Gln double mutation enhanced expression of constitutively active receptors more effectively in HEK 293 cells than in HOS-CD4-Luc cells. This may result from different receptor trafficking in the two cell lines or it may reflect the generally lower transfection efficiency and receptor expression in HOS-CD4-Luc cells. A proposal that the TxP motif acts as a switch that activates CCR5 was supported by mutations that uncoupled the CCR5 receptor from cellular signaling [20,55] or increased constitutive cellular signaling [21]. The Thr2.56(82)Lys and Thr2.56(82)Pro CCR5 mutants that we tested displayed increased basal IP production and could not be further stimulated by MIP-1b. The same mutants were constitutively active and showed 1655472 no further response to chemokine treatment in a yeast reporter system [21], suggesting that they are fully stabilized in activated conformations. They also constitutively stimulated GTPcS binding in stably transfected CHO cells. However, agonist treatment enhanced GTPcS binding [21], suggesting that the Thr2.56(82)Lys and Thr2.56(82)Pro mutations do not fully stabilize the CCR5 conformation that activates the cognate Gai protein. The double mutants, Thr2.56(82)Lys/Arg6.32(225)Gln and Thr2.56(82)Pro/ Arg6.32(225)Gln, both showed basal IP production that was similar to the maximum MIP-1b-stimulated IP production mediated by wild type CCR5, suggesting that they are fully stabilized in activated conformations. However, it is not known whether the CCR5 conformations that activate native Gai signaling pathways are fully stabilized in the double mutant receptors. Mutant receptors with Lys substituted for Thr2.56(82) showed decreased cell surface protein, which may result from decreased receptor stability or stabilization of receptor conformations that constitutively expose cytosolic Ser residues to G protein-coupled receptor kinases, leading to constitutive internalization [44,45,46,47,56]. In contrast, the Thr2.56(82)Pro mutation may stabilize receptor conformations that are not recognized by receptor kinases or are less flexible. The d.
