11 Masked and Translatable Messenger Ribonucleoproteins in Higher Eukaryotes

Abstract

In eukaryotes, production of messenger RNA is not automatically followed by its expression. The cytoplasmic control of mRNA expression seems to be at least as important as the transcriptional and nuclear post-transcriptional levels of regulation. As with DNA in the nucleus, individual mRNA species or whole classes of cytoplasmic mRNA can be in an inactive (“repressed” or “masked”) form or can be active in translation (in polyribosomes). The status of an mRNA presumably is determined by interactions with special cytoplasmic proteins that regulate its translation. Such mRNA-binding proteins include (1) the mRNA-binding initiation factors, (2) specific translational repressors, (3) translational activators, (4) proteins that affect mRNA stability, (5) mRNA masking proteins, and (6) proteins responsible for the structural organization of mRNPs. This chapter focuses mainly on the latter two categories: the masking and structural proteins. Extensive reviews of other mRNA-binding proteins are found elsewhere in this volume: for the translation initiation factors, see Merrick and Hershey and Sonenberg; for factors that promote initiation by binding to IRES elements, see both Jackson and Ehrenfeld; for specific repressors that bind to the 5′ UTR, see Rouault et al. and Meyuhas et al.; for effectors of mRNA stability, see Theodorakis and Cleveland; and for specific regulatory proteins that bind to the 3′ UTR, see Wickens et al., Richter, and Jacobson.

The mRNAs inactive in translation are present in the form of free cytoplasmic mRNP particles. In considering these particles, it is useful to postulate two distinct types of nonactive mRNAs. In the one...