Most land plants can form symbioses with soil-living microbes. The microbes provide nutrients to the plants in return for carbon, in the form of sugars. The plants and microbes need to be able to communicate to enable the microbe to infect into the plant roots. Chemical signals from the microbe activate a signal relay (pathway) in plant cells (Figure 1). Central to this pathway is the activation of nuclear calcium oscillations (repeated increases and decreases in the concentration of calcium ions) and the subsequent activation of a protein called CCaMK.
CCaMK is a very unusual protein because it can bind to calcium ions in two different ways. Firstly, Calcium ions can bind directly to EF-hand domains on the protein. The second way is indirect via the binding of a calcium-binding protein called calmodulin (CaM) to another part of the protein (CaM-binding domain). CCaMK has similarities with the animal CaMKII protein. CaMKII is also activated by calcium oscillations but unlike CCaMK it can only bind calcium via CaM, not directly (it doesn’t have any EF-hands). So why does CCaMK need two ways to bind calcium?
Some of my colleagues at the John Innes Centre are studying CCaMK. They have recently published a paper (1) where they used a combination of biochemistry, genetics, structural modelling and mathematical modelling to understand how CCaMK is activated (interdisciplinary science in action). CCaMK is a kinase, it can attach phosphate groups to amino acids on proteins (phosphorylation). My colleagues found that calcium binding to the EF hands on CCaMK promotes phosphorylation of itself (autophosphorylation) to inactivate CCaMK (see the original paper for data). However, CaM binding to CCaMK overrides the autophosphoryation to activate the protein.
So calcium can both inactivate and activate CCaMK… Confused? Well, it seems that the different calcium-binding mechanisms (to EF-hands or via CaM-binding) operate at different concentrations of calcium, and this is what enables CCaMK to be inactive at low concentrations of calcium (when there are no calcium oscillations) and active at higher calcium concentrations (calcium oscillations).
At low nuclear calcium concentrations there is some calcium bound to the EF-hands of CCaMK so the protein is inactive. When calcium oscillations are activated the calcium concentration in the nucleus rises and calcium ions are more likely to bind to CaM, which then promotes CaM binding to CCaMK. This overrides the autophoshorylation and activates CCaMK, resulting in the switching on of genes required for symbiosis. In this way CCaMK can sense both low and high concentrations of calcium and is a robust switch for symbiosis.
A robust switch would be important because setting up a symbiosis is costly for the plant. The plant would only want to switch on symbiosis genes if the microbe is definitely nearby (and the plant thinks it will be useful!) If CCaMK was not stabilised in an inactive state at low calcium concentrations then a temporary increase in cell calcium concentration could lead to the activation of some of the protein molecules, which could lead to the incorrect switching on of symbiosis genes. Plants with mutant versions of CCaMK that are always active have symbiosis genes switched on even in the absence of a potential microbial partner (2), demonstrating the importance of CCaMK activity in regulating this process.
1) Miller, J.B., et al. (2013) Calcium/Calmodulin-Dependent Protein Kinase Is Negatively and Positively Regulated by Calcium, Providing a Mechanism for Decoding Calcium Responses during Symbiosis Signaling. Plant Cell.
2) Tirichine et al (2006) Deregulation of a Ca2+/calmodulin-dependent kinase leads to spontaneous nodule development. Nature.