REGULATION OF THE EXPORT OF RNA FROM THE NUCLEUS

Transport of macromolecules across the nuclear envelope is an essentia l activity in eukaryotic cells. RNA molecules within cells are found c omplexed with proteins and the bound proteins likely contain signals f or RNA export. RNAs microinjected into Xenopus oocyte nuclei are readi ly exported, and their export can be competed by self RNA but not by R NAs of other classes. This indicates that the rate-limiting step in RN A export is the interaction of RNAs with class-specific proteins, at l east when substrate RNAs are present at saturating levels. Export of h ost mRNAs is inhibited following infection try some animal viruses, wh ile the export of viral RNAs occurs. The HIV-1 RNA-binding protein, Re v, mediates the export of intron-containing viral RNAs that would norm ally be retained in nuclei. This requires a nuclear export signal (NES ) within Rev and an element within the RNA to which Rev binds. In yeas t, heat shock causes accumulation of poly(A)(+) RNA within nuclei but heat-shock mRNAs are transcribed and exported efficiently. This requir es elements within heat shock mRNA that probably interact with a cellu lar protein to facilitate RNA export. In these cases, the proteins tha t recognize critical sequences in the RNAs probably direct the RNAs to an RNA export pathway not generally used for mRNA export. This would circumvent the general retention of most poly(A)(+) mRNAs following he at shock in yeast and the need for complete splicing of viral mRNAs th at travel through the normal mRNA export pathway.