Secretory protein traffic. Chromogranin A contains a dominant targeting signal for the regulated pathway.

Open Access
Publisher Website
Google Scholar
Abstract
Secretory proteins are targeted into either constitutive (secreted upon synthesis) or regulated (stored in vesicles and released in response to a secretagogue) pathways. To investigate mechanisms of protein targeting into catecholamine storage vesicles (CSV), we stably expressed human chromogranin A (CgA), the major soluble protein in human CSV, in the rat pheochromocytoma PC-12 cell line. Chromaffin cell secretagogues (0.1 mM nicotinic cholinergic agonist, 55 mM K+, or 2 mM Ba++) caused cosecretion of human CgA and catecholamines from human CgA-expressing cells. Sucrose gradients colocalized human CgA and catecholamines to subcellular particles of the same buoyant density. Chimeric proteins, in which human CgA (either full-length [457 amino acids] or truncated [amino-terminal 226 amino acids]) was fused in-frame to the ordinarily nonsecreted protein chloramphenicol acetyltransferase (CAT), were expressed transiently in PC-12 cells. Both constructs directed CAT activity into regulated secretory vesicles, as judged by secretagogue-stimulated release. These data demonstrate that human CgA expressed in PC-12 cells is targeted to regulated secretory vesicles. In addition, human CgA can divert an ordinarily non-secreted protein into the regulated secretory pathway, consistent with the operation of a dominant targeting signal for the regulated pathway within the peptide sequence of CgA.