Evidence of a novel dipeptidyl aminopeptidase in mammalian GH(3) cells: new insights into the processing of peptide hormone precursors.
We investigated whether yeast signals could regulate hormone processing in mammalian cells. Chmeric genes coding for the prepro region of yeast alpha-factor and the functional hormone region of anglerfish somatostatin was expressed in rat pituitary GH(3) cells. The nascent prepro-alpha-factor-somatostatin peptides disappeared from cells with a half-life of 30 min, and about 20% of unprocessed precursors remained intracellular after a 2 h chase period. Disappearance of propeptide was insensitive to lysosomotropic agents, but was inhibited at 15 degrees C or 20 degrees C, suggesting that the hybrid propeptides were not degraded in the secretory pathway to the trans Golgi network or in lysosomes. It appeared that while most unprocessed precursors were constitutively secreted into the medium, a small portion were processed at their paired dibasic sites by prohormone-processing enzymes located in trans Golgi network/secretory vesicles, resulting in the production of mature somatostatin peptides. To test this hypothesis, we investigated the processing pattern of two different hybrid precursors: the 52-1 hybrid precursor, which has a Glu-Ala spacer between the prepro region of alpha-factor and somatostatin, and the 58-1 hybrid precursor, which lacks the Glu-Ala spacer. Processing of metabolically labeled hybrid propeptides to smaller somatostatin peptides was assessed by HPLC. When pulse-labeled cells were chased for up to 2 h, 68% of the initially synthesized propeptides were secreted constitutively. About 22% of somatostatin-related products were proteolytically processed to mature somatostatin, of which 38.7% were detected intracellularly after 2 h. From N-terminal peptide sequence determination of somatostatin-related products in GH(3)-52 and GH(3)-58 cells, we found that both hybrid precursors were accurately cleaved at their dibasic amino acid sites. Notably, we also observed that the Glu-Ala spacer sequence was removed from 52-1 hybrid precursors. The latter result strongly suggests that a novel dipeptidyl aminopeptidase activity - a yeast STE13-like enzyme - is present in the post-trans Golgi network compartment of GH(3) cells. The data from these studies indicate that mechanisms which control protein secretion are conserved between yeast and mammalian cells.
Cheong
Kwang Ho
KH
Department of Molecular Biology and Research Center for Cell Differentiation, Seoul National University, Korea.
Lee
Myung Ae
MA
Han
Sang Yeol
SY
Shields
Dennis
D
Park
Sang Dai
SD
Hong
Seung Hwan
SH
D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D002678:Chimera; D004152:Dipeptidyl-Peptidases and Tripeptidyl-Peptidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013006:Growth Hormone / Q000378:metabolism; D008074:Lipoproteins / Q000096:biosynthesis; D010675:Pheromones; D010903:Pituitary Gland, Anterior / Q000166:cytology*; D011498:Protein Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D011499:Protein Processing, Post-Translational; D051381:Rats; D029701:Saccharomyces cerevisiae Proteins; D013004:Somatostatin / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism
D008074:Lipoproteins; C095063:MFA2 protein, S cerevisiae; D010675:Pheromones; D011498:Protein Precursors; D029701:Saccharomyces cerevisiae Proteins; D013004:Somatostatin; D013006:Growth Hormone; D004152:Dipeptidyl-Peptidases and Tripeptidyl-Peptidases