Different Single Receptor Domains Determine the Distinct G Protein Coupling Profiles of Members of the Vasopressin Receptor Family
| Title: | Different Single Receptor Domains Determine the Distinct G Protein Coupling Profiles of Members of the Vasopressin Receptor Family |
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| Authors: | Wess, Jurgen; Liu, Jie |
| Publisher: | Journal of Biological Chemistry |
| Date Published: | April 12, 1996 |
| Reference Number: | 247 |
The vasopressin receptor family is unique among all classes of peptide receptors in that its individual members couple to different subsets of G proteins. The V1a vasopressin receptor, for example, is preferentially linked to G proteins of the Gq/11 class (biochemical response: stimulation of phosphatidylinositol hydrolysis), whereas the V2 vasopressin receptor is selectively coupled to Gs (biochemical response: stimulation of adenylyl cyclase). To elucidate the structural basis underlying this functional heterogeneity, we have systematically exchanged different intracellular domains between the V1a and V2 receptors. Transient expression of the resulting hybrid receptors in COS-7 cells showed that all mutant receptors containing V1a receptor sequence in the second intracellular loop were able to activate the phosphatidylinositol pathway with high efficiency. On the other hand, only those hybrid receptors containing V2 receptor sequence in the third intracellular loop were capable of efficiently stimulating cAMP production. These findings suggest that the differential G protein coupling profiles of individual members of a structurally closely related receptor subfamily can be determined by different single intracellular receptor domains
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This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
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The vasopressin receptor family consists of three receptors: the vasopressin-1a receptor (V1aR), the vasopressin-1b receptor (V1bR), and vasopressin-2 receptor (V2R). Vasopressin (VP), a hormone, is the extracellular messenger that binds with them. When VP binds with V1aR, it affects a different physiological result than when it binds with V1bR or V2R.
G-proteins are a family of proteins located in the membranes that encircle cells. They are coupled to receptors and help them transmit the molecular sequence initiated when the extracellular agent binds with the receptor. The different vasopressin receptors couple to different members of the G-protein family. V1aR and V1bR couple to Gq/11-proteins and V2Rs couple to Gs-proteins. When VP binds with V1aR or V1bR (coupled to Gq/11), it helps activate phospholipase C beta, resulting in the breakdown of phosphoinositide lipids. (This process is known as PI hydrolysis). When VP binds with V2R (coupled to Gs), it activates the enzyme, adenylyl cyclase (AdC).
The structure of the V1R and the V2R is similar. Think of both of them as a beaded string. (The beads are amino_acids. The V2R has 371 of them.) A portion of the receptor lies in 7 distinct folded coils with the cell membrane. These coils are called transmembrane helices 1 - 7. Part of the receptor snakes outside the cell into the extracellular environment to form three curves called extracellular loops 1 - 3. Part of it snakes inside the cell to form three curves called intracellular loops 1 - 3. One end of the receptor, called the amino-terminus, lies outside the cell with the extracellular loops. The other end, called the carboxy-terminus, lies inside the cell with the intracellular loops. (It will be easier to understand this article if you turn to V2R for a visual representation of V2R, which, for purposes of this article, also represents the structure of V21R.)
The vasopressin receptor family is structurally similar, yet its members couple with different G-proteins. What accounts for this? Liu and Wess devised a set of experiments to find out.
They created a series of V1aR/V2R hybrid receptors in which a portion of the V1aR would be substituted for the same portion of the V2R. Thus, the authors constructed a V2R, called V2i1, with a section consisting of the amino-terminus, transmembrane domain 1 and intracellular loop 1 from a V1aR which replaced the V2R's amino-terminus, transmembrane domain 1 and intracellular loop 1. Similarly, they created a V2R with a replacement V1aR intracellular loop 2 (V2i2); a V2R with a replacement V1a intracellular loop 3 (V2i3), and a V2R with a replacement V1a carboxy-terminus.
They also created a V1R with a replacement V2R intracellular 1 loop (V1i1); a V1R with a replacement V2R intracellular loop 2 (V1i2); a V1R with a replacement V2R intracellular loop 3 (V1i3); and a V1R with a replacement V2R carboxy-terminus (V1i4). After creating these hybrids, they expressed each hybrid in cultured cells and measured the hybrids' ability to activate either phosphatidylinositol hydrolysis (which is mediated by the Gq/11-protein) or AVP dependent stimulation of adenylyl cyclase (which is mediated by the Gs-protein).
All the hybrid receptors maintained an ability to bind with VP, and all the hybrid receptors were expressed at levels similar to normal V1a and V2 receptors. However, Liu and Wess observed that the V2i3 receptor (the hybrid receptor that was all V2R except for having the V1a intracellular loop 3) almost completely lost the ability to mediate VP instigated stimulation of AdC.
Likewise, when they created a hybrid V1a receptor that was all V1a except for having a V2R intracellular loop 3, they were able to stimulate cAMP (which is dependent on stimulated AdC). These findings point to the importance to the V2R of its intracellular loop 3.
To study whether or not the various hybrid receptors were capable of coupling to the Gq/11 class, Liu and Wess, measured their ability to help PI hydrolysis to occur. They found that V1aRs in which intracellular loop 1 or 3 or 4 were replaced with the corresponding V2R segment were able to stimulate PI hydrolysis much as the normal V1aR. However, when the V1aR's intracellular loop 2 was replaced with one from a V2R, the PI hydrolysis was weak. Likewise, when intracellular loop 1, 3 or 4 from a V1aR replaced its counterpart in a V2R, the hybrid V2R could not efficiently stimulate PI hydrolysis. Installing a V1A intracellular loop 2 to replace its V2R counterpart resulted in a V2R hybrid capable of stimulating PI hydrolysis just as a V1aR could.
In summary, all the hybrid receptors containing the V1aR sequence in the intracellular 2 loop were able to efficiently activate the PI pathway (mediated by Gq/11), whereas all the hybrid receptors containing V2R sequence in the intracellular 3 loop were able to stimulate AdC (mediated by Gs) Two of the hybrids, V2i2 and V1i3, could activate both the PI and AdC pathways. Thus, the data indicates that different single receptor domains (such as intracellular loop 3) determine which G protein the V1aR and V2R will couple with.