ABC transporters help plants to interact with their environment. ABC transporters are membrane proteins consisting of four modules: two transmembrane domains, which build a tunnel across the membrane, and two nucleotide binding domains, which hydrolyze ATP and thereby energize the transport of different substances across the membrane. This enables the plant to export for example substances against pathogenic microorganisms or herbivores. But is also protects the plant from toxic materials from the soils or helps attracting positive organisms.
Compared to other lifeforms, plants have a significant disadvantage: they cannot move. Thus they always have to deal with the conditions they are exposed to. These can be for example drought or chemically contaminated soils, as well as pathogenic microbes and herbivores. But what can plants do in order to fight back? Plants have an enormous spectrum of secondary metabolites. They produce substances to deter microbial pathogens but also in order to promote growth of beneficial microbes, which for example provide nutrients for the plant.
But how do plants release secondary metabolites into the environment? Every cell is enclosed by a plasma membrane, which separates the cell interior from the surroundings. There is however the problem that the membrane is only partly permeable and many substances cannot pass simply by diffusion. How can plants achieve export of those compounds? Embedded in the membrane are various proteins with different functions. One group of proteins are the ATP binding cassette (ABC) transporters. They are pumps using energy derived from ATP hydrolysis to drive active transport of various substrates across biological membranes.
ABC transporters include two nucleotide-binding domains (NBD) and two transmembrane domains (TMD). The NBDs are highly conserved among all organisms. They bind and hydrolyse ATP, which provides the energy for the transport. The TMDs of ABC transporters are by far less conserved than the NBDs. They form a tunnel across the membrane. They are considered as being important for substrate binding.
In order to better understand the interaction of plants with their environment it is important to study the transported substrates.
In many cases one does only know in what process the membrane protein is involved in, but the exact substrate is still unclear. But how do we find out what is exactly transported? Unfortunately the plant is a complex organism and many processes are tightly linked. Knocking out one gene can thereby influence many downstream processes and lead to unclear results. Therefore we are studying ABC transporters in vitro. To do this we produce the transporters outside the plant and analyse their function in an artificial membrane system.
We will combine our approach with the knowledge of other groups in CEPLAS, which work on other steps of plant secondary metabolite pathways. Together we will be able to better understand plants and their interaction with the environment. In future, the gained knowledge can be transferred to plants, which grow in unfavourable environments.
Contribution by Katharina Gräfe, Institut of Biochemistry I, HHU
Under the heading Planter’s Punch we present each month one special aspect of the CEPLAS research programme. All contributions are prepared by our young researchers.
Ernst R, Kueppers P, Stindt J, Kuchler K, Schmitt L (2010) Multidrug efflux pumps: substrate selection in ATP-binding cassette multidrug efflux pumps—first come, first served? FEBS J 277(3):540-549. [Abstract]
Rea PA (2007) Plant ATP-binding cassette transporters. Annual review of plant biology 58:347-375. [Abstract]