Nucleocytoplasmic transport, protein

Kang, Y. and Cullen, B.R. (1999) The human TAP protein is a nuclear mRNA export factor that contains novel RNA-binding and nucleocytoplasmic transport sequences. Genes Dev., 13,1126-1139. 

Activities of two other enzymes, protein phosphokinease and phospho-protein phosphohydrolase, have also been identified on the mammalian nuclear envelope.113-115 It has been suggested that the levels of phosphorylation and dephosphorylation of the nuclear envelope protein by these two enzymes may regulate nucleocytoplasmic RNA translocation.116 Because these nuclear envelope-associated enzymes may play a key role in the regulation of nuclear RNA transport, a study was conducted to investigate whether the administration of tryptophan would influence the phosphorylation and dephosphorylation process in the hepatic nuclear envelopes, which may then modulate nucleocytoplasmic transport of RNA. The activ-... 

Baglia and Maul158 indicated that a glycoprotein, identified as lamin B, is a major component of the nuclear envelope and nuclear matrix. This glycoprotein may not only be a structural nuclear protein but also may have NTPase activity, which is essential in nucleocytoplasmic transport. Whether changes in the lamin B of liver nuclear envelopes of rats treated with tryptophan occurs is unclear. 

The Ran protein (Ran Ras-related nuclear protein) is an essential component of nucleocytoplasmic transport. There is only one gene for Ran in human cells. During nucleocytoplasmic transport, the Ran protein interacts in a cyclical manner with various import and export receptors, thereby allowing the transport of cargo proteins in and out of the nucleus. An essential feature of the cyclical transport is the asymmetric distribution of the GDP- and GTP-bound forms of Ran between the nucleus and the cytoplasm, which in turn is caused by an asymmetric distribution of GEFs and GAPs for Ran. For a detailed review of Ran, the reader is referred to Kuersten et al, (2001). 

Hamill DR, Suprenant KA. Characterization of the sea urchin major vault protein A possible role for vault ribonucleoprotein particles in nucleocytoplasmic transport. Dev Biol 1997 190(1) 117-28. 

Nehrbass, U., Fabre, E., Dihlmann, S., Herth, W., and Hurt, E. C. (1993). Analysis of nucleocytoplasmic transport in a thermosensitive mutant of nuclear pore protein NSPl. Eur. J. Cell Biol 62, 1-12. 

Most current assays use immunofluorescence or GFP fluorescence to evaluate the nucleocytoplasmic distribution of endogenous nuclear proteins (Corbett et al., 1995 Lim et al, 1995 Schlenstedt et al, 1995) or NLS-containing reporter proteins (Nehrbass et al, 1993 Schlaich and Hurt, 1995) in the steady state. These methods are usually sensitive only to defects that severely impair the transport apparatus (see example below). Even the use of inducible promoters to express reporter proteins after shifting cells to nonpermissive conditions (Corbett et al, 1995 Schlenstedt et al, 1995) requires at least 30-60 min to produce an adequate signal and is, therefore, often too slow. A clever assay was recently described to measure defects in nuclear export signal-mediated protein export in yeast (Lee et al, 1996). An attractive in vivo approach now in use in tissue culture cells is the application of hormone-stimulated GFP-glucocorticoid receptor import to study import kinetics (Htun et al, 1996 Carey et al, 1996). 

Our current understanding of the transport, metabolism, and function of specific RNA molecules has been greatly facilitated by in vitro transcription systems that produce biochemical quantities of pure, radiolabeled or chemically modified RNA molecules. In this chapter we describe procedures that will enable investigators to synthesize specific RNAs, inject theseRNAs into Xenopus oocyte nuclei or cytoplasms, and determine their nucleocytoplasmic distribution, stability, protein associations, and covalent modifications. 

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