Later Stages of Protein Synthesis and the Role of Phospholipids in the Process

Abstract

RECENT work in our laboratory^1,2 has shown that lipo-amino-acid complexes are rapidly formed from amino-acids by protoplasts of B. megaterium, and further that these complexes have many of the properties to be expected of intermediates in protein biosynthesis. Related studies have also been reported by other workers^3–5. We have also found that the membrane complex derived by lysis of the same protoplasts under defined conditions contains the complete apparatus of protein biosynthesis⁶. A more detailed examination⁷ of this membrane complex has now shown that the bulk of the labelled protein formed after very short periods of incubation of the complex with labelled amino-acids (even a few seconds) is distributed between a phospholipoprotein fraction and ribosomal particles. However, the greater part of the labelled protein is found on the phospholipoprotein fraction, and detailed kinetic experiments⁷ carried out with both whole protoplasts and the membrane complex indicate that this is both the main site of initial protein formation and that the protein formed in this fraction is passed on to form soluble protein. In this connexion, it is noteworthy that Dr. P. N. Campbell (personal communication) has evidence indicating that serum albumin, synthesized in microsomes of rat-liver is formed on the membrane rather than the ribosomal component of the microsome, and it has been reported⁸ that a highly labelled phospholipoprotein is rapidly formed from leucine labelled with carbon-14 by ‘ribosomes’ from pea-seedling cells.