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Nuclear Transport

Nuclear Transport

The distinguishing feature of eukaryotic cells is the segregation of ribonucleic acid (RNA) synthesis and deoxyribonucleic acid (DNA) replication in the nucleus , keeping it separate from the cytoplasmic machinery for protein synthesis. As a consequence, messenger RNAs, ribosomal RNAs, transfer RNAs, and all cytoplasmic RNAs of nuclear origin must be transported from their site of synthesis in the nucleus to their final cytoplasmic destinations. Conversely, all nuclear proteins must be imported from the cytoplasm into the nucleus.

Traffic of macromolecules between the nucleus and cytoplasm occurs through nuclear pore complexes (NPCs). NPCs are large proteinaceous structures that form aqueous channels across the nuclear envelope or membrane. NPCs are composed of multiple copies of up to about fifty proteins termed nucleoporins and consist of three structural units. A ringlike central framework surrounding the central channel of the pore is sandwiched between two peripheral structures: the cytoplasmic ring from which eight cytoplasmic fibrils emanate, and the nuclear rim that anchors the nuclear basket.

Nuclear transport depends on signals for import or export that form part of the transported molecules. These signals are referred to as nuclear localization signals (NLSs) or nuclear export signals (NES), respectively. In proteins, they are specific amino acid sequences. NLSs or NESs are recognized and bound by soluble import or export receptors that shuttle between nucleus and cytoplasm. The interaction of the receptors with their cargoes (or substrates) can be direct or mediated by an additional adapter protein. Upon binding, the transport receptors dock their cargoes to the NPC and facilitate their translocation across the central channel of the pore. After delivering their cargoes, the receptors are recycled to initiate additional rounds of transport. According to this model, an export receptor (R) binds its substrate (S) in the nucleus and carries it through the NPC into the cytoplasm. On the cytoplasmic side, the exported cargo is released and the receptor returns to the nucleus without the cargo. Conversely, an import receptor binds its import cargo in the cytoplasm and releases it in the nucleus.

The vast majority of nuclear transport receptors are members of a large family of proteins that exhibit a high affinity for a small GTPase, called Ran, in the GTP bound form. GTP (guanosine triphosphate) is an energy-carrying molecule used in cell signaling. A GTPase like Ran can cause GTP to become GDP (guanosine diphosphate), which will change the properties of the GTPase. The GTPase Ran regulates the interaction of the receptors with their cargoes.

The GTPase acts in concert with several cofactors. The striking property of Ran cofactors is that they are asymmetrically localized in the cell, with some predominantly cytoplasmic while others are predominantly found in the nucleus. This asymmetry helps to control the two-way transport between nucleus and cytoplasm.

see also Membrane Transport; Nucleotides; Nucleus; Protein Targeting; RNA

Elisa Izaurralde

Bibliography

Mattaj, I. W., and L. Englmeier. "Nucleocytoplasmic Transport: The Soluble Phase." Annual Review of Biochemistry (1998) 67: 265-306.

Nakielny, S., and G. Dreyfuss. "Transport of Proteins and RNAs in and Out of the Nucleus." Cell (1999) 99: 677-690.

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RNA Synthesis

RNA Synthesis


Biochemists refer to RNA synthesis as transcription . Transcription is the process of synthesizing ribonucleic acid (RNA). Synthesis takes place within the nucleus of eukaryotic cells or in the cytoplasm of prokaryotes and converts the genetic code from a gene in deoxyribonucleic acid (DNA ) to a strand of RNA that then directs protein synthesis.

Three types of RNA are found in cells. Transfer RNA (tRNA) carries amino acids to the site of protein synthesis. Ribosomal RNA (rRNA) along with protein makes up ribosomes (the mechanism that synthesizes protein). Messenger RNA (mRNA) is the code or template for protein synthesis. Special enzymes synthesize the different forms of RNA.

DNA consists of a series of regions called operons, each containing one or more genes capable of coding for an mRNA strand. An operon consists of a number of segments, principal among which are a promoter region to which RNA polymerase , the enzyme that synthesizes RNA, readily attaches, an operator region that acts as an on/off switch for the operon, and one or more genes that code for mRNA production. For convenience, biochemists describe locations on chains of nucleotides by speaking of the ends of the sugar-phosphate chains as having 3 (three-prime) and 5 ends. Both DNA and RNA are synthesized by enzymes that start at the 5 end of the strand being synthesized.

Transcription begins when RNA polymerase approaches the promoter gene, which often contains extended nucleotide sequences that help to match and bind the polymerase. After the polymerase binds, it is thought to move along the strand of DNA to the operator region. Protein repressor molecules that block transcription bind to operators; inducing agents may attach to the repressor molecules and pull them away from the operator, allowing the synthesis of mRNA. An example of this is the induction or turning on of the lac operon in Escherichia coli by the presence of lactose, producing mRNA that codes for enzymes that metabolize lactose.

RNA polymerase moves along the DNA molecule from the 3 end of the operon to the 5 end, copying only one strand. (Copying the complementary strand would result in useless or harmful nonsense mRNA.) DNA and RNA are similar in composition, but DNA contains deoxyribose instead of ribose and the pyrimidine base thymine instead of uracil . The newly formed RNA is complementary to the DNA code; adenine bases on one strand pair with thymine or uracil on the other strand, and guanine bases on one strand pair with cytosine on the complementary strand. The strands are said to be antiparallel; that is, the 3 end of the DNA strand matches the 5 end of the new mRNA strand. Since protein synthesis (translation) begins at the 5 end of mRNA, protein synthesis in prokaryotic cells can begin while transcription is still under way, increasing the speed with which the organism responds to changes in its environment.

see also Deoxyribonucleic Acid; Protein Synthesis; Ribonucleic Acid.

Dan M. Sullivan

Bibliography

Devlin, Thomas M., ed. (2002). Textbook of Biochemistry: With Clinical Correlations, 5th edition. New York: Wiley-Liss.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.

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