title: Transport of an uncleaved preprotein into the endoplasmic reticulum of rat pituitary cells.
abstract: Preprolactin (Pre-Prl) accumulates isolated rat pituitaries incubated with the threonine analog, beta-hydroxynorvaline. The present study examined whether the failure to cleave Pre-Prl to prolactin inhibits the translocation of the peptide chain across the membrane of the endoplasmic reticulum. In homogenates of pituitary tissue Pre-Prl, prolactin, and growth hormone were not degraded by added proteases. If 0.1% Triton X-100 was added with the proteases these proteins were degraded, suggesting that protection of these proteins from proteases was conferred by membrane vesicles. When Pre-Prl synthesized in a cell-free system was mixed and homogenized with pituitary tissue, it was completely degraded by these enzymes. Thus, protease-resistant Pre-Prl was not the result of nonspecific vesicular entrapment of free Pre-Prl during homogenization. Pre-Prl synthesized in isolated pituitaries copurified with the membrane fraction when the vesicles were isolated from tissue homogenates by flotation in 60% sucrose, and sonication and low concentrations of detergent released Pre-Prl from this fraction. Thus, removal of the prepeptide is not an essential step for the insertion of secretory proteins into the endoplasmic reticulum.
mesh_terms: D000818:Animals; D001692:Biological Transport; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005260:Female; D008861:Microsomes / Q000378:metabolism; D017830:Octoxynol; D010447:Peptide Hydrolases; D010902:Pituitary Gland / Q000378:metabolism*; D011092:Polyethylene Glycols; D011388:Prolactin / Q000378:metabolism*; D011498:Protein Precursors / Q000378:metabolism*; D051381:Rats
chemical_list: D011498:Protein Precursors; D011092:Polyethylene Glycols; C017671:preprolactin; D011388:Prolactin; D017830:Octoxynol; D010447:Peptide Hydrolases