We have identified a novel splice variant of the human and rat two-pore domain potassium (K2P) channel TREK-1. The splice variant TREK-1e results from skipping of exon 5, which causes a frame shift in exon 6. The frame shift produces a novel C-terminal amino acid sequence and a premature termination of translation, which leads to a loss of transmembrane domains M3 and M4 and of the second pore domain. RT-PCR experiments revealed a preferential expression of TREK-1e in kidney, adrenal gland, and amygdala. TREK-1e was nonfunctional when expressed in Xenopus oocytes. However, both the surface expression and the current density of full-length TREK-1 were reduced by co-expression of TREK-1e. Live cell imaging in COS-7 cells transfected with GFP-tagged TREK-1e showed that this splice variant was retained in the endoplasmic reticulum (ER). Attachment of the C-terminus of TREK-1e to two different reporter proteins (Kir2.1 and CD8) led to a strong reduction in the surface expression of these fusion proteins. Progressive truncation of the C-terminus of TREK-1e in these reporter constructs revealed a critical region (amino acids 198 to 205) responsible for the intracellular retention. Mutagenesis experiments indicated that amino acids I204 and W205 are key residues mediating the ER retention of TREK-1e. Our results suggest that the TREK-1e splice variant may interfere with the vesicular traffic of full-length TREK-1 channels from the ER to the plasma membrane. Thus, TREK-1e might modulate the copy number of functional TREK-1 channels at the cell surface, providing a novel mechanism for fine tuning of TREK-1 currents.