In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many.
A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are approximately 1,000 nuclear pore complexes (NPCs) in the nuclear envelope of a vertebrate cell, but it varies depending on cell type and the stage in the life cycle. The human nuclear pore complex (hNPC) is a 110 megadalton (MDa) structure. The proteins that make up the nuclear pore complex are known as nucleoporins; each NPC contains at least 456 individual protein molecules and is composed of 34 distinct nucleoporin proteins.About half of the nucleoporins typically contain solenoid protein domains—either an alpha solenoid or a beta-propeller fold, or in some cases both as separate structural domains. The other half show structural characteristics typical of "natively unfolded" or intrinsically disordered proteins, i.e. they are highly flexible proteins that lack ordered tertiary structure. These disordered proteins are the FG nucleoporins, so called because their amino-acid sequence contains many phenylalanine—glycine repeats.
Protein biosynthesis is a core biological process, occurring inside cells, which balances the loss of cellular proteins through the production of new proteins. Proteins perform a variety of critical functions as enzymes, structural proteins or hormones and therefore, are crucial biological components. As a result, protein synthesis is regulated at multiple steps. Protein synthesis is very similar for prokaryotes and eukaryotes but there are distinct differences.
Alternative splicing, or alternative RNA splicing, or differential splicing, is a regulated process during gene expression that results in a single gene coding for multiple proteins. In this process, particular exons of a gene may be included within or excluded from the final, processed messenger RNA (mRNA) produced from that gene. Consequently, the proteins translated from alternatively spliced mRNAs will contain differences in their amino acid sequence and, often, in their biological functions. Notably, alternative splicing allows the human genome to direct the synthesis of many more proteins than would be expected from its 20,000 protein-coding genes.
A spliceosome is a large and complex molecular machine found primarily within the nucleus of eukaryotic cells. The spliceosome is assembled from small nuclear RNAs (snRNA) and approximately 80 proteins. The spliceosome removes introns from a transcribed pre-mRNA, a type of primary transcript. This process is generally referred to as splicing. An analogy is a film editor, who selectively cuts out irrelevant or incorrect material from the initial film and sends the cleaned-up version to the director for the final cut.
A transfer RNA is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length, that serves as the physical link between the mRNA and the amino acid sequence of proteins. tRNA does this by carrying an amino acid to the protein synthetic machinery of a cell (ribosome) as directed by a 3-nucleotide sequence (codon) in a messenger RNA (mRNA). As such, tRNAs are a necessary component of translation, the biological synthesis of new proteins in accordance with the genetic code.
SR proteins are a conserved family of proteins involved in RNA splicing. SR proteins are named because they contain a protein domain with long repeats of serine and arginine amino acid residues, whose standard abbreviations are "S" and "R" respectively. SR proteins are ~200-600 amino acids in length and composed of two domains, the RNA recognition motif (RRM) region and the RS domain. SR proteins are more commonly found in the nucleus than the cytoplasm, but several SR proteins are known to shuttle between the nucleus and the cytoplasm.
A primary transcript is the single-stranded ribonucleic acid (RNA) product synthesized by transcription of DNA, and processed to yield various mature RNA products such as mRNAs, tRNAs, and rRNAs. The primary transcripts designated to be mRNAs are modified in preparation for translation. For example, a precursor mRNA (pre-mRNA) is a type of primary transcript that becomes a messenger RNA (mRNA) after processing.
snRNPs, or small nuclear ribonucleoproteins, are RNA-protein complexes that combine with unmodified pre-mRNA and various other proteins to form a spliceosome, a large RNA-protein molecular complex upon which splicing of pre-mRNA occurs. The action of snRNPs is essential to the removal of introns from pre-mRNA, a critical aspect of post-transcriptional modification of RNA, occurring only in the nucleus of eukaryotic cells. Additionally, U7 snRNP is not involved in splicing at all, as U7 snRNP is responsible for processing the 3′ stem-loop of histone pre-mRNA.
Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribed by either RNA polymerase II or RNA polymerase III. Their primary function is in the processing of pre-messenger RNA (hnRNA) in the nucleus. They have also been shown to aid in the regulation of transcription factors or RNA polymerase II, and maintaining the telomeres.
In cell biology, a paraspeckle is an irregularly shaped compartment of the cell, approximately 0.2-1 μm in size, found in the nucleus' interchromatin space. First documented in HeLa cells, where there are generally 10-30 per nucleus, paraspeckles are now known to also exist in all human primary cells, transformed cell lines and tissue sections. Their name is derived from their distribution in the nucleus; the "para" is short for parallel and the "speckle" refers to the splicing speckles to which they are always in close proximity. Their function is not known, but they may localize proteins in the nucleus.
U1 spliceosomal RNA is the small nuclear RNA (snRNA) component of U1 snRNP, an RNA-protein complex that combines with other snRNPs, unmodified pre-mRNA, and various other proteins to assemble a spliceosome, a large RNA-protein molecular complex upon which splicing of pre-mRNA occurs. Splicing, or the removal of introns, is a major aspect of post-transcriptional modification, and takes place only in the nucleus of eukaryotes.
The U4 small nuclear Ribo-Nucleic Acid is a non-coding RNA component of the major or U2-dependent spliceosome – a eukaryotic molecular machine involved in the splicing of pre-messenger RNA (pre-mRNA). It forms a duplex with U6, and with each splicing round, it is displaced from the U6 snRNA in an ATP-dependent manner, allowing U6 to re-fold and create the active site for splicing catalysis. A recycling process involving protein Brr2 releases U4 from U6, while protein Prp24 re-anneals U4 and U6. The crystal structure of a 5′ stem-loop of U4 in complex with a binding protein has been solved.
Non-POU domain-containing octamer-binding protein (NonO) is a protein that in humans is encoded by the NONO gene.
Splicing factor 3B subunit 3 is a protein that in humans is encoded by the SF3B3 gene.
Polypyrimidine tract-binding protein 1 is a protein that in humans is encoded by the PTBP1 gene.
Serine/arginine-rich splicing factor 1 (SRSF1) also known as alternative splicing factor 1 (ASF1), pre-mRNA-splicing factor SF2 (SF2) or ASF1/SF2 is a protein that in humans is encoded by the SRSF1 gene. ASF/SF2 is an essential sequence specific splicing factor involved in pre-mRNA splicing. SRSF1 is the gene that codes for ASF/SF2 and is found on chromosome 17. The resulting splicing factor is a protein of approximately 33 kDa. ASF/SF2 is necessary for all splicing reactions to occur, and influences splice site selection in a concentration-dependent manner, resulting in alternative splicing. In addition to being involved in the splicing process, ASF/SF2 also mediates post-splicing activities, such as mRNA nuclear export and translation.
An exon junction complex (EJC) is a protein complex which forms on a pre-messenger RNA strand at the junction of two exons which have been joined together during RNA splicing. The EJC has major influences on translation, surveillance and localization of the spliced mRNA. It is first deposited onto mRNA during splicing and is then transported into the cytoplasm. There it plays a major role in post-transcriptional regulation of mRNA. It is believed that exon junction complexes provide a position-specific memory of the splicing event. The EJC consists of a stable heterotetramer core, which serves as a binding platform for other factors necessary for the mRNA pathway. The core of the EJC contains the protein eukaryotic initiation factor 4A-III bound to an adenosine triphosphate (ATP) analog, as well as the additional proteins Magoh and Y14.The binding of these proteins to nuclear speckled domains has been measured recently and it may be regulated by PI3K/AKT/mTOR signaling pathways. In order for the binding of the complex to the mRNA to occur, the eIF4AIII factor is inhibited, stopping the hydrolysis of ATP. This recognizes EJC as an ATP dependent complex. EJC also interacts with a large number of additional proteins; most notably SR proteins. These interactions are suggested to be important for mRNA compaction. The role of EJC in mRNA export is controversial.
David L. Spector is a cell and molecular biologist best recognized for his research on gene expression and nuclear dynamics. He is currently a professor at Cold Spring Harbor Laboratory (CSHL) and head of the Gene Regulation and Cell Proliferation program of the CSHL Cancer Center. Since 2007, he has served as director of research of CSHL.
Messenger RNP is mRNA with bound proteins. mRNA does not exist "naked" in vivo but is always bound by various proteins while being synthesized, spliced, exported, and translated in the cytoplasm.
