Involvement of free ribosomes in the early stage of secretory protein biosynthesis in rat liver.

Nascent peptides on free and bound ribosomes prepared from rat liver were labeled and released with [3H]puromycin, and the amounts of the nascent peptides of two secretory proteins, serum albumin and transferrin, were determined by immunoprecipitation with specific antibodies. An appreciable amount of serum albumin nascent peptides was associated with free ribosomes, although most of them were carried by bound ribosomes. Serum albumin nascent peptides associated with free ribosomes were smaller in size than those carried by bound ribosomes. When the products of cell-free translation programmed by free and bound polysomes were immunoprecipitated with antibodies, the percentages of serum albumin to the total translation products were in good agreement with those obtained in the nascent peptides experiment. Cell-free translation of free polysomes produced serum albumin with a similar molecular weight to the authentic one. Although nascent peptides of transferrin were detected only on bound ribosomes, and not on free ribosomes, the transferrin antibody immunoprecipitated 0.2% of total translation products of free polysomes. These observations indicate that free polysomes have complete messenger RNA's and shorter growing nascent peptides of serum albumin and transferrin.