Transport of mRNA from Nucleus to Cytoplasm

Abstract

Publisher Summary Transport of mRNP (messenger ribonucleoprotein) from nucleus to cytoplasm plays an important role in gene expression in eukaryotic cells. This chapter focuses on energy-(ATP)-dependent mRNP transport. Nucleocytoplasmic transport of ribosomal RNA can also be induced by ATP, but also occurs by varying [Ca 2+ ]:[Mg 2+ ]. Release of ribosomal RNPs seems to be accompanied by an expansion of the nucleus. Nucleocytoplasmic transport of mRNA seems to be also distinct from the export of tRNA or the exchange of snRNPs and proteins across the nuclear envelope. Nucleocytoplasmic transport of tRNA seems to involve a facilitated diffusion mechanism, showing saturability and sequence specificity; apparently, it does not depend on ATP. In contrast to the transport of mRNPs through the nuclear pore, which appears strictly vectorial, snRNPs can shuttle between nuclear and cytoplasmic compartments. The nuclear uptake of at least some kinds of U-snRNAs in oocytes seems to depend on their association with proteins stockpiled in the cytoplasm. In contrast to the mRNA export, the import of most proteins into the nucleus seems to be energy-independent, although in some cases nucleotides promote this process. The accumulation of karyophilic proteins in the nucleus may be mediated by specific signal sequences recognizing the intranuclear binding sites of these proteins. However, some proteins seem to migrate into the nucleus via a transport mechanism.