IDENTIFICATION OF PROTEIN-PROTEIN INTERACTIONSYeast Two Hybrid System Department of Biotechnology Speaker - HinaOjha
Proteomics is the large-scale study of proteins, particularly their structures and functions. Proteins are vital parts of living organisms, as they are the main components of the physiological metabolic pathways of cells.
Functional proteomics Critical aspects required to understand the function of a protein include:
Protein sequence and structure–used to discover motifs that predict protein function
Expression profile–reveals cell-type specificity and how expression is regulated
Intracellular localization–may allude to the function of the protein
Interactions with other proteins-function may be extrapolated by knowing the function of binding partners.
Why we are studying protein-protein interactions?
Until the late 1990's, protein function analyses mainly focused on single proteins.
But because the majority of proteins interact with other proteins for proper function, they should be studied in the context of their interacting partners to fully understand their function.
With the development of the field of proteomics, understanding how proteins interact with each other and identifying biological networks is vital to understanding how proteins function within the cell.
Biological effects of protein-protein interactions
Alter the kinetic properties of enzymes, which may be the result of subtle changes in substrate binding or allosteric effects
Allow for substrate channeling by moving a substrate between domains or subunits, resulting ultimately in an intended end product
Create a new binding site, typically for small effector molecules
Why Yeast Two-Hybrid system The yeast two hybrid system has a clear advantage over classicalbiochemical or genetic methods
It is an in vivo technique that uses the yeast cell as a living test-tube.
It bears a greater resemblance to higher eukaryotic systems than a system based on a bacterial host.
With regards to classical biochemical approaches, which can require high quantities of purified proteins or good quality anti-bodies, the two hybrid system has minimal requirements to initiate screening, since only the cDNA of the gene of interest is needed.
In signaling cascades, weak and transient interactions are often very important. Such interactions are more readily detected with the two hybrid system since the reporter gene response often leads to significant amplification.
The two hybrid assay is also useful for analysis of known interactions, which can be achieved by modifying important residues or modules and observing this effect on binding.
Interactions can be measured semi-quantitatively using the two hybrid system, allowing discrimination between high, intermediate, and low-affinity bindings, the power of which correlates with that of in vitro approaches
Two hybrid screens are sometimes termed "functional screens", since if at least one of the proteins screened has a known function in a well-defined pathway, it might provide a functional hint in the current interaction.
Although there are certain disadvantages involving the two hybrid assay, the most convincing argument for its use is the speed and ease by which the molecular mechanisms of many signaling cascades have been defined using this technique.
An Introduction to Yeast Saccharomycescerevisiae is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much likeE.coli as the model prokaryote. Saccharomycescerevisiae cells are round to ovoid, 5–10 micrometers in diameter. It reproduces by a division process known as budding S. cerevisiae was the first eukaryoticgenome that was completely sequenced. (April 1996) The genome is composed of about 13,000,000bp and 6,275 genes, although only about 5,800 of these are believed to be true functional genes.
The classical yeast two-hybrid system The early yeast two-hybrid systems were based on the finding that many eukaryotic transcription factors have separable DNA-binding and transcription activation domains.
If X & Y interact then reporter gene is expressed
Major applications of classical system Used to determine whether two known proteins interact with one another Used to identify unknown proteins, encoded by a cDNA library, that interact with a protein of interest Powerful tool for investigating the network of interactions that form between proteins involved in particular biological processes
Selection of host strain The yeast strains used for two-hybrid experiments carry mutations in a number of genes required for amino acid biosynthesis, such as TRP1, LEU2, HIS3 and URA3. If these amino acids are omitted from the growth medium the yeast strain will fail to grow. Many of the two-hybrid plasmids carry genes that complement these mutations and allow for selection of the transformant yeast.
Plasmid encoding X fused in-frame to (Ras GEF) hSos
Plasmid encoding Y fused in-frame with a v-Srcmyristylation signal
Procedure Two plasmids are constructed, one encoding protein X fused in-frame to the human Rasguanyl nucleotide exchange factor (Ras GEF) hSos, the second encoding protein Y fused in-frame with a v-Srcmyristylation signal. The plasmids are co-transformed into a temperature-sensitive yeast mutant containing a lesion in the Cdc25 gene which encodes a yeast Ras GEF. The protein Y fusion is targeted to the yeast plasma-membrane. Interaction between proteins X and Y recruits hSos to the yeast plasma membrane where it complements the cdc25 mutation by activating the Rassignalling cascade. Interactions are detected through growth of yeast cells at the restrictive temperature (37 ◦C).
Applications Cytoplasm-based yeast two-hybrid system Do not rely on a transcriptional readout Transcriptional repressors Auto-activation of reporter genes by bait constructs The problem of certain proteins not localizing to the yeast cell nucleus
Procedure This system is based on the classical yeast two-hybrid system. Proteins X and Y are expressed in-frame with a transcription factor DNA-binding domain and transcription activation domain, respectively. A third protein, Z, is expressed with a nuclear localization signal, without any added domains, in the yeast nucleus. Protein Y may only interact with X in the presence of Z. (i) A domain formed through the interaction between X and Z may provide an interaction interface for protein Y. (ii) Alternatively, protein Z may act as a bridge between proteins X and Y
The dual-bait system X1 fused to DNA-binding domain LexA X2 fused to DNA-binding domain λcI
Procedure Two test proteins (X1 and X2) are fused to two different DNA-binding domains (LexA and λcI, respectively). The two fusion constructs are co-expressed in the same yeast cell and tested for interaction with proteins fused to the B42 transcription activation domain. Interaction with X1 induces expression of LexA-dependent reporter genes (lacZ and LEU2). Interaction with X2 induces expression of λcI-dependent reporter genes (LYS2 and gusA)
Applications Two test proteins can be analyzed for protein-protein interaction partners in a single library screening To test the specificity of a protein-protein interaction amongst evolutionarily conserved proteins To identify domains or residues required for interaction with one partner but not another
Procedure This system is based on the classical two-hybrid system except that expression of the reporter gene is toxic to the yeast cells under certain conditions. In this example, the reporter gene URA3 allows for selection of protein-protein interactions between X and Y on media minus uracil and counter-selection of disrupted protein-protein interaction between X and Y on media containing 5FOA.
Applications This system can be used to identify residues required for protein-protein interaction by making use of a mutagenised copy of the cDNA encoding one of the proteins. cDNAs encoding proteins no longer able to interact can be sequenced to reveal amino acids essential for interaction.