Nuclear exporting sequence

The NHR contains also the conserved Calcineurin docking site, PxlxIT, required for the physical interaction of NEAT and Calcineurin. Dephosphorylation of at least 13 serines residues in the NHR induces a conformational change that exposes the nuclear localization sequences (NLS), allowing the nuclear translocation of NEAT. Rephosphorylation of these residues unmasks the nuclear export sequences that direct transport back to the cytoplasm. Engagement of receptors such as the antigen receptors in T and B cells is coupled to phospholipase C activation and subsequent production of inositol triphosphate. Increased levels of inositol triphosphate lead to the initial release of intracellular stores of calcium. This early increase of calcium induces opening of the plasma membrane calcium-released-activated-calcium (CRAC) channels,... 

Expoitins are transport receptors at the nuclear pore complex needed for the selective export of proteins from the nucleus into the cytoplasm. They recognize nuclear export signal sequences of cargo proteins. 

The Xenopus transcription factor IIIA not only acts as an essential RNA polymerase transcription factor for the expression of the 5S rRNA gene, it also binds to the 5S rRNA to form a 7S ribonucleoprotein particle that stabilizes the RNA until it is required for ribosome assembly and facilitates nuclear export of the 5S rRNA. Indeed, it was originally shown to be the protein component associated with 5S rRNA in the 7S particle in Xenopus oocytes before it was recognized as a transcription factor. How, we may ask, can this protein not only recognize specific DNA sequences in the 5S rRNA gene upstream region, but also recognize different, but equally specific, sequences in 5S rRNA ... 

The Rev protein of HIV-1 facilitates the nuclear export of incompletely spliced viral mRNAs and plays, therefore, an important role in the production of viral structural proteins. Rev specifically binds to a responsive element which folds into a stem-internal loop-stem secondary structure, the Rev-binding element (RBE) located into the rev gene. In vitro selection has been used to determine interactions between Rev and the RBE. RNA motifs which could bind Rev up to ten-fold better than the wild-type sequence have been isolated either from an RNA library constituted of partly randomized RBE (Bartel et al., 1991) or from completely random sequence pools, based on the RBE secondary structure (Giver et al., 1993 Tuerk and MacDougal-Waugh, 1993). Novel RNA sequences and secondary structural motifs have been selected. In particular, a wild-type G G pair is frequently replaced by an A A or even by a C A pair which are isosteric (Giver etal., 1993). 

Bogerd, H.P., Fridell, R.A., Benson, R.E. and Cullen, B.R. (1996) Protein sequence requirements for function of the human T-cell leukemia virus type 1 Rex nuclear export signal delineated by an in vivo randomization-selection assay. Mol. Cell. Biol., 16, 4207-A214. 

Malim, M.H., Hauber, J., Le, S.Y., Maizel, J.V. and Cullen, B.R. (1989b) The HIV-1 rev trans-activator acts through a structured target sequence to activate nuclear export of unsphced viral mRNA. Nature, 338,254-257. 

Proteins which act in the nucleus require specific sequences, known as nuclear localization sequences, to direct their transport from the cytoplasm to the nucleus. The nuclear localization sequences are generally found at the C-terminus of a protein and often comprise basic amino acids. Phosphorylation in sequences that are required for import into or export from the nucleus can decide whether the transcriptional activator is located predominantly in the cytoplasm or in the nucleus where it can exert its activating function. 

Rev protein is accumulated, multiple Rev proteins bind the RRE sequence C Rev activity produces the nuclear export of unspliced or single spliced transcripts, and the gag, pol, env, vif, vpu and vpr proteins are synthesized. 

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