Mechanism of Constitutive Activation of the AT1 Receptor: Influence of the Size of the Agonist Switch Binding Residue Asn111

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• 作者：Ying-Hong Feng ; Shin-ichiro Miura ; Ahsan Husain ; and Sadashiva S. Karnik
• 刊名：Biochemistry
• 出版年：1998
• 出版时间：November 10, 1998
• 年：1998
• 卷：37
• 期：45
• 页码：15791 - 15798
• 全文大小：98K
• 年卷期：v.37,no.45(November 10, 1998)
• ISSN：1520-4995

文摘

The AT₁ receptor is a G-protein-coupled receptor (GPCR); its activation from the basal state(R) requires an interaction between Asn¹¹¹ in transmembrane helix III (TM-III) of the receptor and theTyr⁴ residue of angiotensin II (Ang II). Asn¹¹¹ to Gly¹¹¹ mutation (N111G) results in constitutive activationof the AT₁ receptor (Noda et al. (1996) Biochemistry, 35, 16435-16442). We show here that replacementof the AT₁ receptors TM-III with a topologically identical 16-residue segment (Cys¹⁰¹-Val¹¹⁶) from theAT₂ receptor induces constitutive activity, although Asn¹¹¹ is preserved in the resulting chimera, CR18.Effects of CR18 and N111G mutations are neither additive nor synergistic. The conformation(s) inducedin either mutant mimics the partially activated state (R'), and transition to the fully activated R* conformationin both no longer requires the Tyr⁴ of Ang II. Both the R state of the receptor and the Tyr⁴ Ang IIdependence of receptor activation can be reinstated by introduction of a larger sized Phe side chain at the111 position in CR18, suggesting that the CR18 mutation generated an effect similar to the reduction ofside chain size in the N111G mutation. Consistently in the native AT₁ receptor, R' conformation isgenerated by replacement with residues smaller but not larger than the Asn¹¹¹. However, size substitutionof several other TM-III residues in both receptors did not affect transitions between R, R', and R* states.Thus, the property responsible for Asn¹¹¹ function as a conformational switch is neither polarity norhydrogen bonding potential but the side chain size. We conclude that the fundamental mechanismresponsible for constitutive activation of the AT₁ receptor is to increase the entropy of the key agonist-switch binding residue, Asn¹¹¹. As a result, the normally agonist-dependent R R' transition occursspontaneously. This mechanism may be applicable to many other GPCRs.