STORAGE OF MESSENGER-RNA IN EUKARYOTES - ENVELOPMENT WITH PROTEIN, TRANSLATIONAL BARRIER AT 5' SIDE, OR CONFORMATIONAL MASKING BY 3' SIDE

Messenger RNA can be stored in the cytoplasm of higher Eukaryotes in t he form of masked messenger ribonucleoprotein particles (masked m RN P s, or informosomes). The typical example is the storage of mRNPs in ge rm cells (oocytes and spermatocytes). The masked mRNPs are inactive in translation, stable, i.e., protected against degradation, and unavail able for poly(A) tail processing, such as cytoplasmic polyadenylation and deadenylation. The major nonspecific mRNA-binding protein forming mRNPs and belonging to a special p50 family of basic, glycine-rich, ph osphorylatable proteins seems to be necessary, but not sufficient for the masking. In some cases, mRNA-specific repressor proteins bound to the 5'-untranslated regions (5'-UTR) of mRNAs may be involved. Interac tions of the 3'-untranslated regions (3'-UTR) with sequence-specific p roteins seem to be of decisive importance for the masking of mRNPs. Th e hypothesis is proposed that the masking is achieved through a 3'-UTR -induced conformational rearrangement of mRNP; closing into a circle a nd condensation of mRNP are considered plausible. (C) 1994 Wiley-Liss, Inc.