Angiotensin IV interacts with a juxtamembrane site on AT(4)/IRAP suggesting an allosteric mechanism of enzyme modulation

Angiotensin IV (Ang IV), the 3-8 fragment of angiotensin II, binds to a specific receptor (AT(4)) that has recently been identified as the transmembrane aminopeptidase insulin-regulated aminopeptidase (IRAP) based on the fact that the two proteins share several pharmacological and biochemical properties. Our binding studies indicated that bovine heart expresses relatively large amounts (1.2 pmol/mg protein) of high-affinity binding sites for Ang IV (K(d)=1.8 nM). A photoaffinity-labeling approach combined with mild trypsin digestion revealed that the AT(4) receptor of bovine heart is a single transmembrane domain protein (153 kDa) with a large extracellular fragment (143 kDa). After alkaline denaturation of the AT(4) receptor, trypsin digestion produced two small membrane-associated fragments (16.9 and 6.6 kDa). These results suggest that Ang IV interacts with a juxtamembrane domain of AT(4) receptor. The location of the juxtamembrane site of contact was different from that of the active site of IRAP, suggesting that Ang IV uses an allosteric mechanism to modulate the activity of the AT(4)/IRAP.