Reversible calcium inhibition of the membrane-dependent cleavage of pre-placental lactogen in ascites cell-free extracts

Processing of precursor proteins by preparations of oviduct microsomes

The signal peptidases from both chicken oviduct and dog pancreas microsomes have been detected by a post-translational assay and, in the case of dog microsomes, by a cotranslational assay. This proteolytic activity is not sensitive to a variety of conventional protease inhibitors under the conditions used in these assays. Using a synthetic ester substrate, two additional activities have been observed in oviduct microsomes, but neither appears to correspond to the signal peptidase activity. Using cDNA as a probe for specific mRNAs, it was shown that polysomes, in the process of synthesizing secretory proteins, bind to microsomal membrane vesicles. This binding appears to be dependent on the nature of the translation product (presumably the specific signal peptide) rather than on the mRNA itself. In addition, pretreatment of the membrane vesicles with N-ethyl maleimide prevents the vectorial discharge and processing of several secretory proteins but does not inhibit the binding of their polysomes to membranes.

Inhibition of preprotein processing in ascites tumor lysates by incorporation of a leucine analog

Leucine analogs were tested in the Krebs II ascites cell-free translation system for the ability to inhibit preprotein cleavage by replacing leucine in nascent chains of bovine preprolactin, rat preprolactin, human placental prelactogen (pre-hPL), and pre-alpha subunit of human chorionic gonadotropin (alpha-hCG). In the absence of analog, ascites microsomal membranes cleaved these preproteins to their mature forms and sequestered the processed products. Also, two asparagine residues in alpha-hCG were glycosylated. When 4 mM beta-DL-hydroxyleucine was added to the lysate instead of L-leucine, cotranslational processing and sequestration of both species of preprolactin and pre-hPL were inhibited. Sequential Edman degradation confirmed that pre-hPL was not cleaved. The inhibition of processing by beta-hydroxyleucine resulted from its incorporation into protein. This was shown by reversal of the effect by addition of leucine and by inhibition of [(3)H]leucine incorporation into protein. Of significance, the processing of pre-alpha-hCG was less sensitive to beta-hydroxyleucine because its prepeptide contains only four scattered leucine residues, whereas the presegments of hPL and the prolactins contain six to eight clustered leucine residues. These experiments demonstrate that translocation and processing of secretory proteins require structural features determined by the primary amino acid sequence.