TREK-1, a member of the two-pore-domain potassium channel family, is regulated by polyunsaturated fatty acids, mechanical stretch of the membrane, intracellular acidification and other physical and chemical stimuli. Alternative splicing gives rise to a splice variant, denoted TREK-1e, that lacks the last two trans¬membrane domains due to skipping of exon 5. The alternative splicing produces a frame shift and gives rise to a novel C-terminus. When this C-terminal domain was attached to the reporter proteins Kir2.1 or CD8 a pronounced reduction in surface expression was observed. Successive truncation of the novel C-terminus revealed a novel peptide motif which appeared to be responsible for the retention. Expression analysis in seven different tissues revealed that rat TREK-1e is mainly transcribed in the brain and in the adrenal gland. Immunostaining of TREK-1e expressed in COS-7 cells showed that the protein was localised to the endoplasmic reticulum. Co-expression of TREK-1e with TREK-1a-d in Xenopus oocytes resulted in reduced current amplitudes for TREK-1a-d. A similar mechanism may contribute to the fine-tuning of surface expression if TREK-1 in vivo. We conclude that TREK-1e co-assembles with other TREK-1 splice variants and causes intracellular retention. The mode of action of the novel sorting signal remains to be investigated.