A signal sequence is sufficient for green fluorescent protein to be routed to regulated secretory granules.

To investigate trafficking in neuroendocrine cells, green fluorescent protein (GFP) tags were fused to various portions of the preproneuropeptide Y (NPY) precursor. Two neuroendocrine cell lines, AtT-20 corticotrope tumor cells and PC-12 pheochromocytoma cells, along with primary anterior pituitary cells, were examined. Expression of chimeric constructs did not disrupt trafficking or regulated secretion of endogenous ACTH and prohormone convertase 1 in AtT-20 cells. Western blot and immunocytochemical analyses demonstrated that the chimeric constructs remained intact, as long as the Lys-Arg cleavage site within preproNPY was deleted. GFP was stored in, and released from, regulated granules in cells expressing half of the NPY precursor fused to GFP, and also in cells in which only the signal sequence of preproNPY was fused to GFP. Thus, in neuroendocrine cells, entering the lumen of the secretory pathway is sufficient to target GFP to regulated secretory granules.

Activation and routing of membrane-tethered prohormone convertases 1 and 2.

Many peptide hormones and neuropeptides are processed by members of the subtilisin-like family of prohormone convertases (PCs), which are either soluble or integral membrane proteins. PC1 and PC2 are soluble PCs that are primarily localized to large dense core vesicles in neurons and endocrine cells. We examined whether PC1 and PC2 were active when expressed as membrane-tethered proteins, and how tethering to membranes alters the biosynthesis, enzymatic activity, and intracellular routing of these PCs. PC1 and PC2 chimeras were constructed using the transmembrane domain and cytoplasmic domain of the amidating enzyme, peptidylglycine alpha-amidating monooxygenase (PAM). The membrane-tethered PCs were rerouted from large dense core vesicles to the Golgi region. In addition, the chimeras were transiently expressed at the cell surface and rapidly internalized to the Golgi region in a fashion similar to PAM. Membrane-tethered PC1 and PC2 exhibited changes in pro-domain maturation rates, N-glycosylation, and in the pH and calcium optima required for maximal enzymatic activity against a fluorogenic substrate. In addition, the PC chimeras efficiently cleaved endogenous pro-opiomelanocortin to the correct bioactive peptides. The PAM transmembrane domain/cytoplasmic domain also prevented stimulated secretion of pro-opiomelanocortin products in AtT-20 cells.

PC8 [corrected], a new member of the convertase family.

A novel subtilisin-like protein, PC8, was identified by PCR using degenerate primers to conserved amino acid residues in the catalytic region of members of the prohormone convertase family. PC8 was predicted to be 785 residues long and was structurally related to the mammalian convertases furin, PACE4, PC1 and PC2, sharing more than 50% amino acid identity over the catalytic region with these family members. PC8 possessed the catalytically important Asp, His, Asn and Ser amino acids, the homo B domain of this family of enzymes and a C-terminal hydrophobic sequence indicative of a transmembrane domain. Structurally, PC8 is more related to furin and PACE4 than to PC1 or PC2. Like furin and PACE4, PC8 mRNA was found to be widely expressed; this is in contrast with PC1 and PC2, which have a restricted distribution. Two transcripts, of 4.5 and 3.5 kb, were detected in both human cell lines and rat tissues. Unlike furin and PACE4, both of which map to chromosome 15, PC8 maps to chromosome 11q23-11q24, suggesting that this gene may have resulted from an ancient gene duplication event from either furin or PACE4, or conversely that these genes arose from PC8.