TARGETING OF PROOPIOMELANOCORTIN TO THE REGULATED SECRETORY PATHWAY MAY INVOLVE COOPERATION BETWEEN DIFFERENT PROTEIN DOMAINS

The structure of pro-opiomelanocortin (POMC) can be divided into three main domains: an NH2-terminal domain formed by the NH2-terminal glyco peptide and the joining peptide, a central domain corresponding to the adrenocorticotropin sequences and a COOH-terminal domain containing t he beta-lipotropin sequences. Expression of POMC in neuroendocrine cel l lines such as the mouse neuroblastoma Neuro2A cells results in its t argeting to the regulated secretory pathway of these cells. Intracellu lar targeting of proteins along non default pathways are widely believ ed to involve the recognition of specific structural features by a sor ting machinery. To understand the nature of the signal involved in tar geting prohormone to the regulated secretory pathway, we have construc ted mutants of POMC in which sequences from the NH2-terminal, the cent ral and the COOH-terminal domains were deleted and examined the sortin g of these mutant POMC molecules in Neuro2A cells by immunofluorescenc e and immunoelectron microscopy. Our results indicate that POMC NH2-te rminal glycopeptide or beta-LPH domain do not contain sufficient infor mation for targeting to the regulated pathway since these peptides are not sorted to secretory vesicles when expressed in Neuro2A cells: Sim ilarly, the ACTH domain does not contain essential targeting informati on since POMC mutants lacking these sequences were sorted to secretory vesicles. Mutant POMCs containing-the sequences of more than one of t he main protein domains were, however, correctly targeted to the regul ated secretory pathway. Our results indicate that POMC is not targeted to the regulated secretory pathway through recognition of a unique co ntinuous 'molecular address'. Rather, they suggest that targeting info rmations are present in different domains of POMC and that cooperation between these domains is needed for sorting of the precursor to the r egulated secretory pathway.