Human ACE and bradykinin B2 receptors form a complex at the plasma membrane

To investigate how angiotensin I-converting enzyme ( ACE) inhibitors enhance the actions of bradykinin (BK) on B-2 receptors independent of blocking BK inactivation, we expressed human somatic ACE and B-2 receptors in CHO cells. Bradykinin and its ACE-resistant analog were the receptor agonists. B-2 fused with green fluorescent protein (GFP) and ACE were coprecipitated with antisera to GFP or ACE shown in Western blots. Immunohistochemistry of fixed cells localized ACE by red color and B-2-GFP by green. Yellow on plasma membranes of coexpressing cells also indicated enzyme-receptor complex formation. Using ACE-fused cyan fluorescent protein donor and B-2-fused yellow fluorescent protein (YFP) acceptor, we registered fluorescence resonance energy transfer (FRET) by the enhanced fluorescence of donor on acceptor photobleaching, establishing close (within 10 nm) positions of B-2 receptors and ACE. Bradykinin stimulation cointernalized ACE and B-2 receptors. We expressed ACE fused to N terminus of B-2 receptors, anchoring only receptors to plasma membranes. Here, in contrast to cells, where both ACE and B-2 receptors are separately anchored, ACE inhibitors neither enhance activation of chimeric B-2 nor resensitize desensitized B-2 receptors. Heterodimer formation between ACE and B-2 receptors can be a mechanism for ACE inhibitors to augment kinin activity at cellular level.