The role of carboxypeptidases in the intracellular processing and sorting of neuroendocrine peptides

Abstract

Carboxypeptidase E (CPE) is involved with the processing of numerous neuroendocrine peptides. CPE has been implicated in the regulation of body weight and fertility in mice; mice homozygous for the fat mutation (fat/fat) which introduces Pro in place of Ser{dollar}\sp{lcub}202{rcub}{dollar} of CPE develop obesity and hyperglycemia. CPE has been also proposed to act as a universal sorting receptor directing hormones into the regulated pathway. Despite the lack of CPE activity, fat/fat mice have a partial ability to process neuroendocrine peptides, suggesting another carboxypeptidase is involved in peptide processing along with CPE.;To confirm that the Ser{dollar}\sp{lcub}202{rcub}{dollar} to Pro mutation was responsible for the absence of CPE activity in the fat/fat mouse, this mutation was created in rat CPE, and the enzymatic properties and the fate of this mutant were examined. The results indicate that mutant CPE is inactive and degraded prior to transport into the Golgi. Using cell lines derived from the beta cells of fat/fat mice, we demonstrated that in spite of the reduced ability to produce mature insulin, these cells are able to secrete insulin/proinsulin in the regulated fashion, indicating that CPE is not obligatory for the sorting of proinsulin into the regulated pathway. We also demonstrated that carboxypeptidase D is located in the trans-Golgi network, and that it is recycled from the cell surface to the trans-Golgi network, which is consistent with a role for this enzyme in the processing of neuroendocrine peptides as well as other proteins that transit the secretory pathway.