Plant & zo
The science of plants and more
Another genius bit of biological design comes from the enzyme PLC. PLC is a phospholipase whose job it is to cleave of the headgroup of PI(4,5)P2, a membrane lipid. Like tethering proteins, PLCs are made up out of a couple of domains. At its core there are three domains. There is the catalytic domain, which is doing all the action. Then there is a lipid binding domain, C2 domain, whose job it is to attach the protein to the membrane, preferably near PI(4,5)P2. Last there is a EF-hand domain, which likes to interact with other proteins. The order of these domains in the proteins is as follow: EF-hand domain – catalytic domain – C2 domain. Mammalian PLCs may have a couple of other domains, so called regulatory domains, on either end. But plant PLCs don’t so it is unknown how exactly their activity is regulated.
However, there have been some studies done on which domains are required to be there for which action, and how the protein looks when it is all folded up. First its structure, in contrast to its sequence order, when folded up the C2 domain sits in the middle. With on one side the catalytic domain, to which it is loosely attracted to. On the other side is the EF-hand domain to which it feels quite some attraction so it huddles close to it. By doing so it stretches the linker between the EF-hand domain and the catalytic domain, shoving the catalytic domain close to the C2 domain. In doing so, when bound to the membrane, it places the catalytic domain just right so it can grab hold of a PI(4,5)P2 headgroup to cleave it off.
This by itself is just brilliant already. Looking at what is required for what function, expectantly they found that for PLCs membrane localization its C2 domain was just enough. However, looking at what is needed to chop of the headgroup of PI(4,5)P2 things became puzzled. It turned out that with just the catalytic domain, nothing happens, likely because the membrane could not be found. But having the catalytic and the C2 domain is not enough either. The EF-hand domain needs to be there, holding them all close together, so the catalytic domain can find PI(4,5)P2.
This suggests that when the EF-hand domain is not bound to the C2 domain, the catalytic domain is just hanging of the C2 domain, away from the membrane, unable to do its job. Implying that controlling the interaction between the C2-domain and the EF-hand domains is a way of regulating PLC’s activity.
Realizing this was quite an eyeopener for me. However, what surprised me more was that although this was all discovered over 15 years ago. No subsequent research seem to have been done to find out the details of how the interaction between the C2 domain and EF-hand domain is regulated.
Otterhag L, Sommarin M, Pical C. N-terminal EF-hand-like domain is required for phosphoinositide-specific phospholipase C activity in Arabidopsis thaliana. FEBS Lett. 2001;497(2-3):165-170. doi:10.1016/s0014-5793(01)02453-x
Jiménez JL, Smith GR, Contreras-Moreira B, et al. Functional recycling of C2 domains throughout evolution: a comparative study of synaptotagmin, protein kinase C and phospholipase C by sequence, structural and modelling approaches. J Mol Biol. 2003;333(3):621-639. doi:10.1016/j.jmb.2003.08.052