The mechanosensitive K_2P-channel TREK-1 is involved in many important physiological processes and is regulated by diverse physical and chemical stimuli and several protein interaction partners. Using the split-ubiquitin system, a variant of the yeast two-hybrid method, we identified the enzyme heme oxygenase 2 (HO-2) as a possible interaction partner of TREK-1 in human tissues. Functional interaction between TREK-1 and HO-2 was studied by co-expressing TREK-1 with HO-2 in Xenopus oocytes and measuring the resulting changes in current amplitude. Co-expression of HO-2 resulted in a significant increase in TREK-1 current. The surface expression of TREK-1, measured with an enzyme-coupled luminometric assay, was not altered after co-expression of HO-2. Thus the increase in current amplitude was probably due to a change in the open-state probability of the channel. HO-2 catalyses the breakdown of heme into biliverdin, iron and carbon monoxide (CO). To study the influence of the heme degradation product CO on TREK-1 a CO-donor was applied. The CO-donor elicited an increase in TREK-1 current in both Xenopus oocytes and HEK 293 cells. Mutation of either of the two serine residues at amino acid positions 311 and 344 drastically reduced the stimulatory effect of CO on current amplitude, indicating that C-terminal TREK-1 phosphorylation sites may be involved in CO dependent activation of TREK-1 currents. Our results suggest that an interaction between HO-2 and TREK-1 leads to increased local production of CO, which activates the channel.