Polyribosomes plasma proteins

B. Plasma Proteins Are Generally Synthesized ON Membrane-Bound Polyribosomes ... 

Most proteins that are synthesized on membrane-bound polyribosomes and are destined for the Golgi apparatus or plasma membrane reach these sites inside transport vesicles. The precise mechanisms by which proteins synthesized in the rough ER are inserted into these vesicles are not known. Those involved in transport from the ER to the Golgi apparatus and vice versa—and from the Golgi to the plasma membrane— are mainly clathrin-free, unlike the coated vesicles involved in endocytosis (see discussions of the LDL receptor in Chapters 25 and 26). For the sake of clarity, the non-clathrin-coated vesicles will be referred to in... 

Many proteins synthesized on membrane-bound polyribosomes proceed to the Golgi apparatus and the plasma membrane in transport vesicles. 

Moja et al.57 treated rats with cycloheximide, an inhibitor of protein synthesis, and then fed a tryptophan-free mixture. The cycloheximide treatment largely prevented the marked decrease of plasma tryptophan (total and free). These data supported the hypothesis that protein synthesis was the mechanism through which the ingested tryptophan-free mixture decreased blood tryptophan levels. However, this interpretation was not consistent with the findings of Wunner et al.58 and Sidransky et al.,59 who observed a low rate of polyribosome formation and low incorporation of labeled amino acids in the livers of rodents previously fasted and then acutely fed a tryptophan-free amino acid mixture. 

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