Transport of an uncleaved preprotein into the endoplasmic reticulum of rat pituitary cells.

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.