Endothelin-1 (ET-1) is one of the most potent peptide vasoconstrictors known. It is produced upon the cleavage of its precursor Big Endothelin-1 by Endothelin Converting Enzyme-1 (ECE-1).Production of ET-1 is thought to be dependent upon the expression of ECE-1 at the cell surface. Therefore mechanisms inducing the trafficking of ECE-1 to the cell surface havebeen the focus of recent research.This research has identified phosphorylation of the cytoplasmic region of ECE-1as a main cellular signal inducing its trafficking to the cell surface. Previous studies have used green fluorescent protein (GFP) tagged ECE-1 to monitor phosphorylation induced trafficking of ECE-1 to the cell surface. However, it has been speculated that the addition of the GFP tag can itself alter enzyme activity and phosphorylation of ECE-1, and hence the suitability of GFP or any other protein tag in studying ECE-1 distribution and trafficking. ECE-1c is the most widely expressed isoform in endothelial cells. We thereforeexpressedECE-1c with a GFP tag either at the N or C-terminus of ECE-1c.Catalytic activity and effect on Protein Kinase C (PKC) induced phosphorylation was compared between the two chimerasand wild type ECE-1c. Our results indicate that positioning of the GFP tag on the C-terminus abrogates activity without effecting PKC induced phosphorylation. However, GFP tag on the N-terminus has the opposite effect. Results of this study shed light on the applicability of GFP or perhaps other protein tags in studying ECE-1c distribution and trafficking.