Corin, a Transmembrane Cardiac Serine Protease, as a Pro-Atrial Natriuretic Peptide Convertase
Atrial natriuretic peptide (ANP) is a cardiac hormone that reduces high blood pressure by promoting salt excretion, decreasing blood volume and relaxing vessel tension. It is implicated in major cardiovascular diseases such as hypertension and congestive heart failure. In cardiomyocytes, ANP is synthesized as a precursor, pro-ANP, that is converted to biologically active ANP by an unknown membrane-associated serine protease. Recently, we cloned a novel cardiac serine protease, corin, that has the predicted structure of a type II transmembrane protein. Northern and in situ hybridization detected corin mRNA in the heart where its expression was most abundant in cardiomyocytes of the atrium. Corin mRNA was also detected in developing kidney and bones. The overall expression pattern of corin mRNA was very similar to that of ANP, leading to our hypothesis that corin is the pro-ANP convertase. To test this hypothesis, we constructed expression vectors for both human corin and pro-ANP. Recombinant corin and pro-ANP were expressed in human 293 cells. Effects of corin on pro-ANP processing was examined by western analysis. The results showed that human corin, but not control proteases prothrombin and hepsin, converted pro-ANP to ANP in the cell-based experiments. To determine sequence specificity of corin-mediated cleavage in pro-ANP, we constructed a mutant pro-ANP R98G in which the predicted cleave site residue Arg98 was replaced with a Gly. In co-transfection experiments, corin processed wild-type pro-ANP but not mutant pro-ANP R98G, indicating that corin-mediated cleavage in pro-ANP is highly sequence specific. We also examined effects of protease inhibitors on processing of pro-ANP by corin. Our results showed that the activity of corin was inhibited by aprotinin, benzamidine and leupeptin but not soybean trypsin inhibitor. Similar effects of these proteases on pro-ANP processing were reported previously. Thus, corin matches all known characteristics of the long-sought pro-ANP convertase. Identification of corin as the pro-ANP convertase suggests a new regulatory mechanism for the ANP-mediated pathway that is important for controlling blood pressure.