MHC (major histocompatibility complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I); hence, the MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway
Synthesis
During synthesis, MHC class II is the result of dimerization of α and β chains, with the assistance of an invariant chain.[2] The invariant chain is a special polypeptide involved in the formation and deliverance of MHC class II protein.
The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. This fuses with a late endosome containing the endocytosed, degraded proteins. It is then broken down in stages, leaving only a small fragment called CLIP which still blocks the peptide binding cleft. An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome. The stable MHC class-II is then presented on the cell surface.
The SH2 (Src Homology 2) domain is a structurally conserved protein domain contained within the Src oncoprotein and in many other intracellular signal-transducing proteins.Protein-Protein interactions play a major role in cellular growth and development. Modular domains, which are the subunits of a protein, moderate these protein interactions by identifying short peptide sequences. These peptide sequences determine the binding partners of each protein. One of the more prominent domains is the SH2 domain. SH2 domains play a vital role in cellular communication. Its length is approximately 100 amino acids long and it is found within 115 human proteins. Regarding its structure, it contains 2 alpha helices and 7 beta strands. Research has shown that it has a high affinity to phosphorylated tyrosine residues and it is known to identify a sequence of 3-6 amino acids within a peptide motif.
SH2 domains typically bind a phosphorylated tyrosine residue in the context of a longer peptide motif within a target protein, and SH2 domains represent the largest class of known pTyr-recognition domains.
Phosphorylation of tyrosine residues in a protein occurs during signal transduction and is carried out by tyrosine kinases. In this way, phosphorylation of a substrate by tyrosine kinases acts as a switch to trigger binding to an SH2 domain-containing protein. The intimate relationship between tyrosine kinases and SH2 domains is supported by their coordinate emergence during eukaryotic evolution.
SH2 domains are not present in yeast and appear at the boundary between protozoa and animalia in organisms such as the social amoeba Dictyostelium discoideum.
A detailed bioinformatic examination of SH2 domains of human and mouse reveals 120 SH2 domains contained within 115 proteins encoded by the human genome, representing a rapid rate of evolutionary expansion among the SH2 domains.
Function
The function of SH2 domains is to specifically recognize the phosphorylated state of tyrosine residues, thereby allowing SH2 domain-containing proteins to localize to tyrosine-phosphorylated sites. This process constitutes the fundamental event of signal transduction through a membrane, in which a signal in the extracellular compartment is "sensed" by a receptor and is converted in the intracellular compartment to a different chemical form, i.e. that of a phosphorylated tyrosine. Tyrosine phosphorylation leads to activation of a cascade of protein-protein interactions whereby SH2 domain-containing proteins are recruited to tyrosine-phosphorylated sites. This process initiates a series of events which eventually result in altered patterns of gene expression or other cellular responses. Protein with at least one Src homology 2 (SH2) domain. The SH2 domain, which was first identified in the oncoproteins Src and Fps, is about 100 amino-acid residues long. It functions as a regulatory module of intracellular signaling cascades by interacting with high affinity to phosphotyrosine-containing target peptides in a sequence-specific and strictly phosphorylation-dependent manner
RNA polymerase II (also called RNAP II and Pol II) is an enzyme found in eukaryotic cells. It catalyzes the transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA. A 550 kDa complex of 12 subunits, RNAP II is the most studied type of RNA polymerase. A wide range of transcription factors are required for it to bind to its promoters and begin transcription.
Stages of transcription
In the process of transcription (by any polymerase) there are three main stages:
1. Initiation; the construction of the RNA polymerase complex on the gene's promoter with the help of transcription factors.
2. Elongation; the actual transcription of the majority of the gene into a corresponding RNA sequence, highly moderated by several methods.
3. Termination; the cessation of RNA transcription and the disassembly of the RNA polymerase complex.
Due to the range of genes Pol II transcribes this is the polymerase that experiences greatest regulation, by a range of factors, at each stage of transcription. It is also one of the most complex in terms of polymerase cofactors involved.
Initiation
Preinitiation complex (PIC): the construction of the polymerase complex on the promoter. The TATA box is one well-studied example of a promoter element. It is conserved in many (though not all) model eukaryotes and is found in a fraction of the promoters in these organisms. The sequence TATA is located at approximately 25 nucleotides upstream of the Transcription Start Point (TSP). In addition, there are also some weakly conserved features including the TFIIB-Recognition Element (BRE), approximately 5 nucleotides upstream (BREu) and 5 nucleotides downstream (BREd) of the TATA box.
Order in which the GTFs attach
The following is the order in which the GTFs (general transcription factors) attach:
1. TBP (TATA Binding Protein) and an attached complex of TAFs (TBP Associated Factors), collectively known as TFIID (Transcription Factor for polymerase II D), bind at the TATA box.†
2. TFIIA (three subunits) binds TFIID and DNA, stabilizing the first interactions.
3. TFIIB binds between TFIID and the location of Pol II binding in the near future. TFIIB binds partially sequence specifically, with some preference for BRE.
4. TFIIF (two subunits, RAP30 and RAP74, showing some similarity to bacterial sigma factors) and Pol II enter the complex together. TFIIF helps to speed up the polymerization process.
5. TFIIE enters the complex, and helps to open and close the Pol II’s ‘Jaw’ like structure, which enables movement down the DNA strand. TFIIE and TFIIH enter concomitantly.
6. Finally TFIIH and TFIIJ to the complex together. TFIIH is a large protein complex that contains among others the CDK7/cyclin H kinase complex and a DNA helicase. TFIIH has three functions: it binds specifically to the template strand to ensure that the correct strand of DNA is transcribed and melts or unwinds the DNA (ATP dependently) to separate the two strands using its Helicase activity. It has a kinase activity that phosphorylates the C-terminal domain (CTD) of Pol II at the amino acid serine. This switches the RNA polymerase to start producing RNA, which marks the end of initiation and the start of elongation. Finally it is essential for Nucleotide Excision Repair (NER) of damaged DNA. TFIIH and TFIIE strongly interact with one another. TFIIE affects TFIIH’s catalytic activity. Without TFIIE, TFIIH will not unwind the promoter.
7. Mediator then encases all the transcription factors and the Pol II. Mediator interacts with enhancers, areas very far away (upstream or downstream) that help regulate transcription.
Initiation Regulation
Initiation is regulated by many mechanisms. These can be separated into two main categories:
1. Protein interference.
2. Regulation by phosphorylation.
Regulation by Protein interference
Protein interference is the process where some signaling protein interacts, either with the promoter or some stage of the partially constructed complex, to prevent further construction of the polymerase complex, so preventing initiation. This is generally a very rapid response and is used for fine level, individual gene control and for 'cascade' processes for a group of genes useful under a specific conditions (for example DNA repair genes or heat shock genes)
Chromatin structure inhibition is the process where the promoter is hidden by chromatin structure. Chromatin structure is controlled by post-translational modification of the histones involved and leads to gross levels of high or low transcription levels. See: chromatin, histone and nucleosome.
These methods of control can be combined in a modular method, allowing very high specificity in transcription initiation control.
This year's Nobel Prize in Chemistry awards Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath for having showed what the ribosome looks like and how it functions at the atomic level. All three have used a method called X-ray crystallography to map the position for each and every one of the hundreds of thousands of atoms that make up the ribosome.
Inside every cell in all organisms, there are DNA molecules. They contain the blueprints for how a human being, a plant or a bacterium, looks and functions. But the DNA molecule is passive. If there was nothing else, there would be no life.
The blueprints become transformed into living matter through the work of ribosomes. Based upon the information in DNA, ribosomes make proteins: oxygen-transporting haemoglobin, antibodies of the immune system, hormones such as insulin, the collagen of the skin, or enzymes that break down sugar. There are tens of thousands of proteins in the body and they all have different forms and functions. They build and control life at the chemical level.
An understanding of the ribosome's innermost workings is important for a scientific understanding of life. This knowledge can be put to a practical and immediate use; many of today's antibiotics cure various diseases by blocking the function of bacterial ribosomes. Without functional ribosomes, bacteria cannot survive. This is why ribosomes are such an important target for new antibiotics.
This year's three Laureates have all generated 3D models that show how different antibiotics bind to the ribosome. These models are now used by scientists in order to develop new antibiotics, directly assisting the saving of lives and decreasing humanity's suffering.
