Carboxypeptidase E is efficiently secreted and internalized via lysosomes

Carboxypeptidase E (CPE) a key factor in the biosynthesis of most peptide hormones and neuropeptides, is predominantly expressed in endocrine tissues and the nervous system. This highly conserved enzyme cleaves the C-terminal basic residues of the peptide precursors to generate their bioactive form. CPE is a secreted protein; however, the Intracellular pathways leading to its secretion are still obscure. We combined live-cell microscopy and molecular analysis to examine the intracellular distribution and secretion dynamics of fluorescently tagged CPE. CPE was found to be a soluble luminal protein as it traffics from the ER via the Golgi apparatus to lysosomes. Moreover, CPE is efficiently secreted and reinternalized to lysosomes of neighboring cells. The C-terminal amphipathic helix of CPE is essential for its efficient targeting to, and secretion from lysosomes. Fluorescence resonance energy transfer demonstrated that CPE and its substrate neuropeptide Y (NPY) interact in the Golgi apparatus and Immunoprecipitation analysis demonstrated that both CPE and NPY are co-secreted. The implications of the well-defined CPE intra and extracellular routes are discussed.