- retrieve a STRING network for a protein of interest
- interpret the different visual representations
- inspect the evidence behind an interaction
- understand the query parameters and customize searches
- retrieve a STITCH network for chemical compound of interest
- use DISEASES to find proteins associated with a disease of interest
1.1 Single protein query
We will first retrieve a STRING network for human insulin receptor (INSR). Go to https://string-db.org/, open the Protein by name search interface, and type INSR in the field Protein Name. You can either specify Homo sapiens in the Organism field or leave it on auto-detect. Click SEARCH. You will first be presented with a disambiguation page, on which you can specify that the protein you are interested in is the human protein INSR, and press Continue to view the protein network.
1.2 Visual representations
The STRING web interface provides several different visual representations of the network. The Settings tab below the network view allows you to change between different visual representations of the same network. Try changing between the confidence and evidence views; do not forget to press the UPDATE button.
Which information is shown for the edges in each representation? Why are there sometimes multiple lines connecting the same two proteins in the evidence representation?
1.3 Evidence viewers
A key feature of the STRING web interface is the evidence viewers. One should not rely purely on the confidence scores; it is important to inspect the actual evidence underlying an interaction before relying on it, for example, for designing experiments.
Which types of evidence support the interaction between insulin receptor (INSR) and insulin receptor substrate 1 (IRS1)?
Further detail on the evidence of an interaction can be seen in a popup by clicking on the corresponding edge in the network. Click on the edge between INSR and IRS1 to view its popup; you may need to move the nodes to make this easier.
Which type of evidence gives the largest contribution to the confidence score 0.999?
Click on the Show button to view the experimental evidence for the interaction.
Which types of experiments support this interaction?
1.4 Query parameters
The Settings tab also allows you to modify detailed parameters for the search, such as the network type, the types of evidence to use (active interaction sources), the minimum required interaction score, and the max number of interactors to show.
Change the minimum required interaction score to high confidence (0.700).
Does this change the set of proteins shown? Does it change the interactions shown?
Change the network type from full STRING network to physical subnetwork.
Does this change the set of proteins and interactions shown in the network? (Hint: check the Legend tab.)
What evidence types are available for the physical interactions? Are there more or fewer evidence types than in the full STRING network?
Increase the max number of interactors to show to 20.
How many interaction partners of INSR do you get in the network? What happens if you increase it to 50?
Change the minimum required interaction score back to 0.400.
How many INSR interactors do you now get? (Hint: check the Analysis tab.)
Switch back to full STRING network.
Does this change the set of proteins shown in the network?
2.1 Chemical compound query
To retrieve for a chemical compound, go to http://stitch-db.org/, type aspirin in the Item Name field, type Homo sapiens in Organism, and click the SEARCH button. You should now see a network of proteins associated with aspirin, including PTGS1 and PTGS2, which are the direct targets of aspirin.
2.2 Binding assay data
Click the interaction between aspirin and PTGS1 and then the Show button next to Experimental/Biochemical Data in the popup. The evidence viewer now shows all the available binding assay data for this interaction, including the binding affinities (IC50 values).
3.1 Disease query
Go to https://diseases.jensenlab.org/, type Parkinson in the search field, and click the search button. The web interface will now show the search results, which include all diseases and protein names starting with the search term. Click Parkinson's disease to get to the results page showing proteins associated with the disease.
Like STRING and STITCH, the DISEASES database integrates several types of evidence, in this case automatic text mining, manually curated knowledge, and experimental evidence from genome-wide association studies.
Are there any proteins that are supported by all three types of evidence?
3.2 Validation of text mining
The DISEASES database too allows you to inspect the underlying evidence for an association. Since the predominant source of evidence is automatic text mining, it is always wise to read the underlying text to manually validate the results. Click on SNCA in the Text mining table to view the text based on which it was associated with the disease. Click View abstract for a given entry to see the complete abstract rather than only the title.
Do the abstracts all mention both the protein and the disease? Do they all use the same name for the protein?
4.1 Complete virus query
Go to http://viruses.string-db.org, and select Complete Virus from the menu on the left. In the Virus dropdown, enter “Measles”, and the Host dropdown can be left as auto-detect to detect the host with the most interactions, in this case, Homo sapiens. Click Search to retrieve the network.
4.2 Inspect virus evidence
Click on the edge connecting the measles virus P/V protein and the human STAT2 protein.
What types of evidence support an interaction between these proteins? List two publications that the evidence comes from.
4.3 Single virus protein query
Click on the logo at the top of the page to go back to the main search screen. Select Virus by Single Protein from the left, and then enter “P” as the Virus Protein Name and “bacteriophage lambda” as the Virus. The Host can again be left to auto-detect E. coli. Protein P is responsible for the bi-directional replication of phage DNA.
Which host proteins does P interact with? What types of evidence supports these interactions?
The theoretical background for these exercises is covered in these short online lectures:
Szklarczyk D, Gable AL, Nastou KC, Lyon D, Kirsch R, Pyysalo S, Doncheva NT, Legeay M, Fang T, Bork P, Jensen LJ and von Mering C (2021). The STRING database in 2021: customizable protein-protein networks, and functional characterization of user-uploaded gene/measurement sets. Nucleic Acids Research, 49:D605–D612.
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