Identification of molecular regions responsible for the membrane trafficking of Kir6.2

The subunits of the pancreatic ATP-sensitive potassium channel Kir6.2 and t he sulphonylurea receptor (SUR1) contain endoplasmic reticulum (ER) retenti on signals (RKR), which prevent their plasma membrane expression when expre ssed individually. When co-expressed, however, these signals are masked and the complex traffics to the plasma membrane. To investigate this further, we have expressed epitope-tagged chimaeras between Kir6.2 and Kir2.1 (which traffics to the membrane independently of SUR1) in Xenopus oocytes alone a nd together with SUR1. By staining sections of the oocytes, we show that, i n addition to the ER retention signal present in the distal C-terminus, the M2 transmembrane and the proximal C-terminal regions also contribute to th e inability of Kir6.2 to traffic to the membrane in the absence of SUR1. Fu rthermore, by staining the whole oocytes for the hexa-histidine tag attache d to the N-terminus of SUR1, we provide direct experimental evidence that t he N-terminus of SUR1 is extracellular.