EVIDENCE FOR THE INTRACELLULAR LOCATION OF CHLORIDE CHANNEL (CIC)-TYPE PROTEINS - COLOCALIZATION OF CIC-6A AND CIC-6C WITH THE SARCO ENDOPLASMIC-RETICULUM CA2+ PUMP SERCA2B/

Chloride channel protein (ClC)-6a and ClC-6c, a kidney-specific splice variant with a truncated C-terminus, are proteins that belong structu rally to the family of voltage-dependent chloride channels. Attempts t o characterize functionally ClC-6a or ClC-6c in Xenopus oocytes have s o far been negative. Similarly, expression of both ClC-6 isoforms in m ammalian cells failed to provide functional information. One possible explanation of these negative results is that ClC-6 is an intracellula r chloride channel rather than being located in the plasma membrane. W e therefore studied the subcellular location of C1C-6 isoforms by tran siently transfecting COS and CHO cells with epitope-tagged versions of ClC-6a and ClC-6c. Confocal imaging of transfected cells revealed for both ClC-6 isoforms an intracellular distribution pattern that clearl y differed from the peripheral location of CD2, a plasma-membrane glyc oprotein. Furthermore, dual-labelling experiments of COS cells co-tran sfected with ClC-6a or -6c and the sarco/endoplasmic-reticulum Ca2+ pu mp (SERCA2b) indicated that the ClC-6 isoforms co-localized with the S ERCA2b Ca2+ pump. Thus ClC-6a and ClC-6c are intracellular membrane pr oteins, most likely residing in the endoplasmic reticulum. In view of their structural similarity to proven chloride channels, ClC-6 isoform s are molecular candidates for intracellular chloride channels.