J. Biol. Chem., Vol. 261, Issue 24, 10953-10956, Aug, 1986
Demonstration of post-translational secretion of human placental lactogen by a mammalian in vitro translation system
MP Caulfield, LT Duong and M Rosenblatt
This study demonstrates the post-translational translocation across the
rough endoplasmic reticular membrane of a mammalian secretory protein,
human preplacental lactogen. In the rabbit reticulocyte lysate, human
preplacental lactogen biosynthesis is arrested by addition of cycloheximide
prior to supplementation with dog pancreatic microsomal membranes, which
have previously been shown to translocate and process nascent secretory
proteins in a cotranslational manner. Twenty-five percent of the precursor
protein is consistently converted to its mature form under these
post-translational conditions. The resulting mature hormone is resistant to
proteolytic degradation by added proteases, thus indicating that it is
translocated across the microsomal membrane and sequestered within the
lumenal space of the microsomal vesicles. Approximately one-half of the
precursor protein synthesized is associated with the ribosomes. Only the
ribosome- associated fraction is secreted in this in vitro system,
suggesting that the process of post-translational secretion requires
ribosomes for protein interaction with the elements of a subcellular
secretory apparatus.