Biosynthesis and processing of ribophorins in the endoplasmic reticulum

The initial observations described above led us to examine growing cultures of rat hepatocytes (cl-9 cells) as possible rich sources of ribophorin-mRNA. After short periods of in vivo labeling with [35S]methionine, newly synthesized ribophorin molecules were easily detected in these cells by immuneprecipitation with specific antibodies. These appeared indistinguishable electrophoretically from those previously characterized in rat liver (Fig. 2, compare lanes a and c and lanes b and d) and therefore cl-9 cells appeared appropriate subjects for biosynthesis studies. Moreover, when added to the in vitro translation system, messenger RNA extracted from these cells directed the synthesis of measurable amounts of both ribophorins (see Fig. 1 , lane c for ribophorin I).