Membrane proteins synthesized but not processed by isolated maize chloroplasts

Abstract

One-dimensional maps of proteolytic fragments generated by digestion with Staphylococcus aureus protease in sodium dodecyl sulfate (SDS) were used to identify three polypeptides synthesized by isolated Zea mays chloroplasts. This technique does not depend upon proper incorporation of the newly synthesized polypeptides into a more complex structure for their identification. The only preliminary purification required is electrophoretic separation on SDS-polyacrylamide gels. The pattern of radioactive fragments from labeled proteins which co-migrate with the alpha and beta subunits of chloroplast coupling factor (CF1) corresponds precisely to the pattern of stainable fragments derived from subunits of the purified enzyme. A 34,500-dalton protein is the major membrane-associated product of protein synthesis by isolated maize chloroplasts. From the similarity in the fragments formed by digestion with S. aureus protease, it appears that this radioactive protein is probably a precursor of a 32,000-dalton protein which is a component of the thylakoid. The alpha and beta subunits of CF1 newly synthesized by isolated chloroplasts are not fully extractable by procedures which normally solubilize the enzyme from membranes. The 34,500-dalton protein is not processed to the 32,000-dalton form in any great amount by isolated chloroplasts. A 19,000-dalton fragment of the 32,000-dalton protein is protected from digestion when thylakoids are treated with proteases, while the newly synthesized 34,500-dalton protein is fully susceptible. The isolated chloroplast does not appear to be able to fully integrate these newly made proteins into the membrane structure.