Cell surface expression of 5-hydroxytryptamine type 3 receptors is controlled by an endoplasmic reticulum retention signal

Two subunits of the 5-hydroxytryptamine type 3 (5-HT(3)) have been identified (5-HT(3A) and 5-HT(3B)) that assemble into homomeric (5-HT(3A)) and heteromeric (5-HT(3A) + 5-HT(3B)) complexes. Unassembled 5-HT(3B) subunits are efficiently retained within the cell. In this study, we address the mechanism controlling the release of 5-HT(3B) from the endoplasmic reticulum (ER). An analysis of chimeric 5-HT(3A) receptor(R).5-HT(3B)R constructs suggests the presence of elements downstream of the first transmembrane domain of 5-HT(3B) subunits that inhibit cell surface expression. To investigate this possibility, truncated 5-HT(3B) subunits were constructed and assessed for their ability to access the cell surface in COS-7 and ts201 cells. Using this approach, we have identified the presence of an ER retention signal located within the first cytoplasmic loop between transmembrane domains I and II of 5-HT(3B). Transplantation of this signal (CRAR) into the homologous region of 5-HT(3A) results in the ER retention of this subunit until rescued by co-assembly with wild-type 5-HT(3A). The mutation of this ER retention signal in 5-HT(3B) (5-HT(3B)SGER) does not lead to cell surface expression, suggesting the presence of other signals or mechanisms to control the surface expression of 5-HT(3B)Rs. The generation of truncated 5-HT(3B)SGER constructs confirmed that the CRAR signal does play an important role in the ER retention of 5-HT(3B).