Role of nuclear pore in transport. The import and export of protejns and RNAs into and outside the nucleus are facilitated by the presence of signal sequences. Even gold particles 25nm in diameter, if coated with a signal sequence—bearing nuclear protein, are transported into and outside the nucleus. ATP has also been found to be essential for nuclear transport. The transport (this is usually unidirectional) actually involves following two steps: (i) an ATP-independent, but signal-sequence dependent step involving binding of protein to the pore, and (ii) an ATP-requiring step, involving translocation through the pore; this step is the only energy requiring step. It has been shown that in the absence of ATP, the protein binds to the pore, but can not be transported to the nucleus.
(a) Protein import and export Nuclear proteins, like other proteins, are synthesized in the cytoplasm, and are later imported into the nucleus. Mostly, these proteins are transported unidirectionally from the cytoplasm to the nucleus and can not exit the nuclear pore, once they are imported. Such proteins include SV40 T antigen and Xenopus nucleoplasmin, which have been studied in some detail. There are few other proteins, which shuttle between nucleus and cytoplasm, either constitutively (irrespective of requirement) or in response to regulatory signals. These include various nuclear proteins like hsc70 and the cAMP dependent protein kinase. It is predicted that these proteins contain an additional signal sequence for protein export. Some of the ribosomai proteins are imported to associate with rRNA, so that the ribosomai subunits are assembled inside the nucleus and then exported through the pore to the cytoplasm, a process, which is ATP-dependent. Although several signal sequences for import have been identified, no signal sequences for export are known. These should be discovered only in future.
Signal sequences that have been identified for many nuclear proteins to be imported, have often been compared with SV40 T antigen sequence, Pro-Lys-Lys-Lys-Arg-Lys-Val. In this sequence, mutation of second lysine to threonine greatly reduces import, suggesting the role of this amino acid in import. Many other signal sequences resemble more closely, the first identified signal sequence, i.e. Xenopus protein nucleoplasmin, whose gene has also been cloned now. The signal sequence of nucleoplasmin is larger (16 amino acids) than that for T-antigen,and has two independent basic domains separated by 10 spacer amino acids. It is now believed that the bipartite nucleoplasmin signal sequence will be the type most often found in other proteins. The existence of nuclear signal sequences predicts that there must also exist one or more receptors for these signals. Some such receptors have been isolated (particularly in yeast) and are believed to be located either in the pore or in the cytoplasm. In the latter case, they bind the signal sequence and carry the protein to the pore.
(b) RNA export and import. One of the most vital roles of nuclear pore is the unidirectional export of RNA, including tRNAs, snRNAs (small nuclear RNAs) and processed mRNA. Ribosomai RNA is used in the nucleus for assembly of small subunits,. which are later exported, as discussed above. Thus, the rRNA and unprocessed mRNA are not allowed to be exported. It has been shown that for export of processed mRNAs and snRNAs, the 5' monomethyl cap acquired during transcription is essential (see Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)). Several cap binding proteins have been identified and may help in RNA export. They may work as RNA export receptors or may contain protein export signal, which helps the export of cap-binding protein-mRNA complex.
Few cellular RNAs are also imported into the nucleus. These include snRNAs (particularly Ul and U2), which are synthesized with monomethyl cap and are first exported to the cytoplasm, to be converted into dimethyl form, which is assembled as snRNPs (small nuclear ribonucleo-proteins). These snRNPs, with the help of a bipartite signal (one for export and other for import), are reimported into the nucleus. T-DNA of Ti plasmid of Agrobacterium tumefaciens (used for gene transfer; see Genetic Engineering and Biotechnology 4. Gene Transfer Methods and Transgenic Organisms) is an example of the import of DNA into the nucleus.
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