The polyribosomal mRNA--protein complex is a dynamic structure.

The metabolism of mRNA--protein complexes present in polyribosomes of mouse L cells was investigated with the aid of ultraviolet light-induced crosslinking of proteins to RNA. It was shown that a set of at least seven proteins which are synthesized and become associated with mRNA under conditions permitting mRNA synthesis also are synthesized and become associated with mRNA when mRNA synthesis is inhibited with a high dose of actinomycin D. This set includes a protein of Mr 78,000 which can be crosslinked to poly(A). These findings imply that mRNA-associated proteins exchange in the cytoplasm with a pool of free proteins. Taken together with results from other laboratories, they suggest a role for mRNA-associated proteins in translation. The labeling kinetics of mRNA-associated proteins were also investigated and found to be consistent with cytoplasmic addition to mRNA and a half-life of more than 2 hr. Additional implications of these findings are discussed.