2004 Global Researcher Conference Proceeding
April 09 - 11, 2004
| Conference: | 2004 Global Researcher Conference |
|---|---|
| Title: | Compartmentalization of NDI-causing vasopressin V2 receptor mutants in the early secretory pathway |
| Authors: | Hermosilla, Ricardo; Rosenthal, Walter; Schulein, Ralf |
| Institutions: | Forschungsinstitut fur Molekulare Pharmakologie (FMP), Charite - Universitatsmedizin Berlin |
The wild-type vasopressin V2 receptor (V2R) is transported via the endoplasmic reticulum (ER), the ER/Golgi intermediate compartment (ERGIC) and the Golgi apparatus to the plasma membrane. In contrast, the majority of the NDI-causing mutations in the V2R gene lead to transport-defective receptors that are retained intracellularly. Mutant receptors are recognized by a quality control system that allows only correctly folded receptors to escape the early secretory pathway. It was previously thought that quality control in the early secretory pathway is restricted to the endoplasmic reticulum (ER).
Here, we have examined the retention mechanisms of eight transport-defective V2R mutants in HEK 293 cells in detail. We show that the mutant receptors can be classified into two groups. The receptor mutants L62P, DL62-R64 and S167L belong to class A. These receptors are trapped exclusively in the ER and never leave this compartment. In contrast, the class B mutants R143P, Y205C, InsQ292, V226E and R337X reach the ERGIC. They are recognized in this compartment and are rerouted to the ER. Next, we have studied the properties of class B mutants allowing them to escape the ER. A plausible hypothesis is that mutants expressed at a very high level saturate the quality control system of the ER and that the ERGIC may serve as a safety net to retain highly expressed malfolded proteins. However, transport studies with stable cell lines expressing different amounts of the mutant receptors exclude this possibility. Another hypothesis implies that the ER escape of a mutant receptor is the consequence of a particular folding state that is only inefficiently recognized in the ER. Limited proteolysis experiments with trypsin strongly support this hypothesis.
In summary, our results demonstrate that the ERGIC contributes to quality control of the V2R and that disease-causing mutants of a membrane protein may be retained in different compartments of the early secretory pathway. Our results may also have implications for all the other diseases where transport-defective membrane proteins play a role.
In its journey from the cell interior to the cell membrane, the vasopressin-2 receptor (V2R) must pass through the following cell structures: first, the endoplasmic reticulum (ER), then the ER/Golgi intermediate compartment (ERGIC), and then the Golgi apparatus. Most mutant V2Rs that result in NDI never reach the cell membrane. This is because the quality control structures of the cells do not allow misshapen V2R proteins to pass through.
Most researchers thought that the V2R quality control structure was the ER. Schulein, et al., discovered a group of mutant V2Rs that made it through the ER to reach the ERGIC. The ERGIC recognized that the members of this group are misshapen and sends them back to the ER. The researchers suggest that the shape of the V2Rs in this group is not clearly recognized by the ER, which is why they can pass through. The researchers have demonstrated that the ERGIC plays a part in the quality control of V2Rs.