Chloroquine and Monensin Alter the Posttranslational Processing and Secretion of Dopamine ?-Hydroxylase and Other Proteins from PC12 Cells

Expression of human dopamine beta-hydroxylase in mammalian cells infected by recombinant vaccinia virus. Mechanisms for membrane attachment

Dopamine beta-hydroxylase (DBH) is found in neurosecretory vesicles in both membrane-bound and soluble forms. We expressed various human DBH cDNAs in two mammalian cell lines, using the vaccinia virus expression system. The expression of a full-length DBH cDNA (DBH-f) reproduced the native DBH electrophoretic pattern and led to the synthesis of an active enzyme composed of two subunits of 77 and 73 kDa. In contrast, a truncated cDNA lacking the first ATG (DBH-t) generated a single band of 73 kDa. Analysis of mutated recombinant clones demonstrates that the two polypeptides do not result from the use of an alternative translation initiator codon. These results, combined with deglycosylation experiments, allow us to attribute the double band pattern to an optional cleavage of the signal peptide. When the NH2-terminal extremity is shortened, cleavage becomes obligatory, underlining the role of the first 14 amino acids in the regulation of the cleavage of the signal peptide. Subcellular analysis of recombinant DBH-t and DBH-f proteins indicates that DBH is anchored to the membrane by two distinct mechanisms; one of them is due to the non-removal of the signal peptide, whereas the second one is independent of the presence of the signal sequence. Moreover, quantification of the fractionation experiments suggests that the two modes of membrane attachment are additive.

Effect of compounds which disrupt proton gradients on secretion of neurosecretory proteins from PC12 pheochromocytoma cells

Treatment of PC12 cells with chroquine (10-50 microM) obliterated the low intragranular pH, as detected by Acridine Orange fluorescence, and depleted the cells of dopamine and norepinephrine. However, these concentrations of chloroquine did not prevent the release of the newly synthesized proteins which normally undergo stimulus-coupled secretion with the catecholamines. Higher concentrations of chloroquine (200 microM) and ammonium chloride (10 and 25 mM) inhibited the release of most of these proteins. This inhibition did not result from alterations in protein synthesis, since the profile of proteins synthesized was not substantially altered. Nor did the inhibition result from degradation of the neurosecretory proteins, since prelabeled proteins were capable of undergoing stimulated secretion from chloroquine-treated cells, as from normal cells. The findings indicated that the inhibition was at the step of packaging of the proteins into the neurosecretory granules. While release of the major secretory proteins, including chromogranin B, was inhibited with 200 microM chloroquine, chromogranin A was secreted upon stimulation of these cells. The results of this study indicate that an acidic intragranular pH is not a requirement for the packaging and secretion of neurosecretory proteins. Higher concentrations of chloroquine had a differential effect on the regulated secretion of different neurosecretory proteins.

Structure of bovine adrenal dopamine beta-monooxygenase, as deduced from cDNA and protein sequencing: evidence that the membrane-bound form of the enzyme is anchored by an uncleaved signal peptide

A full-length cDNA for dopamine beta-monooxygenase (D beta M) from bovine adrenal glands has been cloned and sequenced. The soluble and membrane-derived forms of D beta M have also been sequenced from their N-termini. While the observed sequences for the soluble protein correspond to those previously reported [Joh, T.H., & Hwang, O. (1986) Ann. N.Y. Acad. Sci. 493, 343-350], the heavy subunit of membrane-derived enzyme is found to contain a unique N-terminus. Alignment of this N-terminus with that deduced from cDNA cloning indicates identity at 22 (and possibly 26) out of 27 residues. This comparison leads us to conclude that the membranous form of bovine D beta M retains an uncleaved N-terminal signal peptide as the source of membrane anchoring.

Topogenesis and sorting of synaptophysin: synthesis of a synaptic vesicle protein from a gene transfected into nonneuroendocrine cells

Diverse nonneuroendocrine (non-NE) cells were forced to express synaptophysin (SY), the major and typical transmembrane glycoprotein of small (30-80 nm) neurotransmitter vesicles of NE cells, using microinjection of RNA synthesized in vitro from cDNA or transient and stable transfections with cDNA brought under SV40 promoter control. The glycoprotein synthesized in non-NE cells is indistinguishable from SY of NE cells and is integrated with similar, if not identical, orientation in the membranes of a specific, novel type of small cytoplasmic vesicle that structurally resembles synaptic vesicles and in which SY is the only major protein detected. A non-N-glycosylated form of SY generated by site-directed mutagenesis showed the same behavior and specific distribution in small vesicles. The results show that the information contained in this protein alone is sufficient to secure its sorting into a special type of vesicle in a heterotypic context, i.e., in the absence of other NE-specific components.