The structure of m7GpppN (where N is any nucleotide), termed cap, is present at the 5' end of all eukaryotic cellular mRNAs (except organellar) The eukaryotic initiation factor 4E (eIF-4E) binds to the cap and facilitates the formation of translation initiation complexes eIF-4E is implicated in control of cell growth, as its overexpression causes malignant transformation of rodent cells and deregulates HeLa cell growth It was suggested that overexpression of eIF-4E results in the enhanced translation of poorly translated mRNAs that encode growth-promoting proteins Indeed, enhanced expression of several proteins, including cyclin D1 and ornithine decarboxylase (ODC), was documented in eIF-4E-overexpressing NTH 3T3 cells However, the mechanism underlying this increase has not been elucidated Here, we studied the mode by which eIF-4E increases the expression of cyclin D1 and ODC We show that the increase in the amount of cyclin D1 and ODC is directly proportional to the degree of eIF-4E overexpression Two mechanisms, which are not mutually exclusive, are responsible for the increase In eIF-4E-overexpressing cells the rate of translation initiation of ODC mRNA was increased inasmuch as the mRNA sedimented with heavier polysomes For cyclin D1 mRNA, translation initiation was not increased, but rather its amount in the cytoplasm increased, without a significant increase in total mRNA Whereas, in the parental NIH 3T3 cell line, a large proportion of the cyclin D1 mRNA was confined to the nucleus, in eIF-4E-overexpressing cells the vast majority of the mRNA was present in the cytoplasm These results indicate that eIF-4E affects directly or indirectly mRNA nucleocytoplasmic transport, in addition to its role in translation initiation

https://www.pnas.org/content/pnas/93/3/1065.full.pdf

Translation initiation of ornithine decarboxylase and nucleocytoplasmic transport of cyclin D1 mRNA are increased in cells overexpressing eukaryotic initiation factor 4E

Structure of spliceosomal snRNPs and their role in pre-mRNA splicing.

Identification of a nuclear export receptor for tRNA

The Rev trans-activator of human immunodeficiency virus type 1 (HIV-1) is a protein that regulates the simultaneous appearance in the cytoplasm of both spliced and unspliced forms of viral mRNAs from the same viral transcripts by way of recognition of a target sequence termed the Rev-responsive element (RRE). Whether Rev acts directly on RNA export or by inhibition of splicing, or both, is still a matter of debate. We have addressed this issue in Xenopus laevis oocytes by microinjecting RNA molecules containing RRE along with purified recombinant Rev protein into the oocyte nuclei. Adenovirus pre-mRNA containing an RRE in the intron was spliced equally well in the absence and presence of Rev protein. Only in the presence of Rev was non-spliced pre-mRNA exported from the nucleus; more surprisingly, the excised intron lariat (containing RRE) was also exported. Furthermore, an RRE-containing mRNA molecule that lacked intron sequences was also efficiently exported from the nucleus in a Rev-dependent manner. Therefore our results demonstrate that Rev can act directly at the level of nuclear export, independent of any inhibitory effect that it may exert on the splicing of pre-mRNA. Finally, our finding that the Rev mutant M10, shown previously to be inactive in human lymphoid cells, was also unable to export RRE-containing RNA molecules from oocyte nuclei suggests that one or more cellular factors, evolutionarily conserved between humans and Xenopus, interact with Rev in both cell systems to promote nuclear RNA export.

Evidence that HIV-1 Rev directly promotes the nuclear export of unspliced RNA

Although spliceosomal Sm proteins can assemble spontaneously onto UsnRNA in vitro, this process requires assisting factors in vivo. SMN, the protein involved in spinal muscular atrophy, is part of a complex that contains the Sm proteins and serves as a critical factor for this reaction. Here, we have reconstituted the SMN-dependent assembly of UsnRNPs in vitro. We demonstrate that the SMN complex is necessary and sufficient for the assembly reaction. The PRMT5 complex, previously implicated in methylation and storage of Sm proteins, interacts with the SMN complex and enhances its activity in an ATP-dependent manner. These data uncover the SMN–PRMT5 complex as a functional entity that promotes the assisted assembly of spliceosomal UsnRNPs, and potentially other, RNA–protein complexes.

/pdf/assisted-rnp-assembly-smn-and-prmt5-complexes-cooperate-in-2qz4uflfo8.pdf

Assisted RNP assembly: SMN and PRMT5 complexes cooperate in the formation of spliceosomal UsnRNPs

The particle state of U snRNPs was analyzed in oocytes, eggs, embryos and testes from Xenopus laevis. In each case both the relative abundance and the composition of some U snRNPs were found to differ from that of somatic cells. U2 and U6 snRNPs were the most prominent U snRNPs in germ cells and early embryos. In particular, the concentration of U6 snRNA was 10-20 times higher than that of U4 snRNA. Most of the U6 snRNA was not associated with U4 snRNA and migrated on sucrose gradients as a U6 snRNP. The structure of this novel U snRNP was analyzed. A single protein of 50 kd was copurified with U6 snRNPs by a combination of gradient fractionation, immunodepletion with anti-Sm antibodies and immunoprecipitation with anti-6-methyl adenosine antibodies. Although the U6 snRNP did not contain Sm proteins it migrated into the nucleus when U6 snRNA was injected into the cytoplasm of oocytes. Two U6 snRNA elements have been identified. The first is essential for nuclear migration in oocytes, but not for the formation of U4/6 snRNPs in vitro and might be the binding site of a U6-specific protein. The second element was required for interaction with U4 snRNPs but not for nuclear targeting.

https://www.embopress.org/doi/pdf/10.1002/j.1460-2075.1989.tb08603.x

An abundant U6 snRNP found in germ cells and embryos of Xenopus laevis.

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An abundant U6 snRNP found in germ cells and embryos of Xenopus laevis.