Biology is full of complex, yet efficient, systems. Signaling pathways, for example, have multiple components that often need to be specifically arranged in order to function. As evolution favors efficiency, cells have often adopted components of an already established signaling pathway for a new purpose rather than creating new pathways from scratch. Researchers from Madrid and Glasgow recently used this concept and their knowledge of the organized structure of ciliary signaling to guide their research questions about the signaling pathway organization of T cell immune synapses.
Cilia are organelles that protrude out from the plasma membrane like small antennae. And like antennae, their job is signaling. Cilia organize signaling pathways by making sure every component of a pathway is in the right place by trafficking those molecules to their correct location. Cilia have many specific molecules they use for this trafficking process.
Immune synapses also require specific localization of their signaling components. An immune synapse is the point of contact that forms between the T cell receptor (TCR) of a T cell binding to its specific antigen being presented by an antigen presenting cell. For this information to be carried to the inside of the cell so it can organize an immune response, the TCR must first be phosphorylated.
Phosphorylation happens when a phosphate group is added to a molecule by an enzyme called a kinase. Phosphorylation can activate a molecule or inactivate it, depending on the molecule and the site where it is added. The TCR is phosphorylated by the kinase LCK.
How does LCK know to localize at the immune synapse? And how does it become activated? As it turns out, ciliary proteins hold the key!
Stephen et al. found that the ciliary trafficking protein UNC119A binds specifically to LCK, and not to other kinases of the same family. Specific binding is imperative if the cells need to localize a particular molecule to a particular location. What about UNC119A and LCK allows for this specific binding? The answer lies in their structure.
Structure and function of molecules are inexorably linked, and by learning more about structure, function often becomes clear. By looking at the crystal structure of LCK bound to UNC119A, the researchers determined that LCK binds in a pocket of UNC119A because it has three small amino acids at one particular location. These small amino acids let it nestle into the pocket, whereas other kinase family members with larger amino acids at that location cannot. They checked their findings by making amino acid substitutions. When they replaced a small amino acid in the LCK sequence with a larger one, its ability to bind UNC119A was decreased. When they replaced the larger amino acids of other kinase family members with smaller ones, they found that the kinases bind UNC119A more strongly.
How does the specific binding of UNC119A to LCK affect LCK’s function? Well, the LCK with the large amino acid substitution—in addition to binding UNC119A poorly—also didn’t localize to the immune synapse. Binding of LCK to UNC119A is important for its trafficking to the immune synapse, but for LCK to function at the immune synapse, it needs to be released from UNC119A. How does this happen?
To figure out how LCK is released from UNC119A, researchers looked at another ciliary protein, ARL3. The active form of ARL3 is ARL3-GTP, which localizes to the immune synapse. They found that ARL3-GTP disrupts the binding of LCK to UNC119A by phosphorylating LCK, allowing LCK to complete its function and phosphorylate the TCR.
These discoveries linking ciliary trafficking to immune synapse trafficking through UNC119A and ARL3-GTP have already been found to have relevance to patients. UNC119A is involved in a ciliopathy (a disease that results from a problem with cilia) that leads to problems with vision. A mutation UNC119A has also been reported in a patient with an immunodeficiency disorder called Idiopathic CD4 lymphopenia which leads to fewer T cells.
When trying to figure out the components of a complex biologic system, it can often be helpful to look at similar systems for ideas. Cilia and immune synapses both rely on specific localization of signaling molecules, so it’s no surprise that there’s overlap with the proteins involved in each of these systems! By utilizing information that’s already known about one system, we can more easily put together puzzle pieces of knowledge about another system, leading to faster advances in understanding and treating diseases.
Stephen, L.A.,et al. (2018). The Ciliary Machinery is Repurposed for T Cell Immune Synapse Trafficking of LCK.Developmental Cell. 47(1):122-132. doi:10.1016/j.devcel.2018.08.01
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Transmission Lines by Chris Hunkeler CC BY-SA 2.0