27.04.2017

Whenever the concentration of Calcium ions rises rapidly inside the cell, it functions as a universal signal. The signal's details are encoded in magnitude and time course of the concentration change. Thus, external stimuli are often translated into this type of secondary signal: the release of Calcium ions (Ca²⁺). A well-known example is the release of Ca²⁺ in muscle cells after a nerve impulse leading to muscle contraction. But plants, too, use Ca²⁺ as a second messenger, for instance, upon environmental stresses as in the case of a pathogen attack. Inside a cell, a variety of Calcium-binding proteins sense the rise of the second messenger and pass on the signal. Among these proteins are the Calmodulins (CaMs), which bind Ca²⁺ ions and subsequently interact with other signalling proteins that finally activate specific responses. The family of CaM-binding proteins, in turn, is large and heterogenous and they are crucial in translating the Ca²⁺ signal into specific responses including growth, development, adaptation, or defense responses in a plant cell.

Katharina Bürstenbinder with her research group Cellular Coordination focuses on studying one large family of CaM-binding proteins, so-called IQ-Domain (IQD) proteins. Their latest results have recently been published in the journal Plant Physiology (Bürstenbinder, K., Möller, B., Plötner, R., Stamm, G., Hause, G., Mitra, D. & Abel, S. The IQD Family of Calmodulin-Binding Proteins Links Calcium Signaling to Microtubules, Membrane Microdomains, and the Nucleus, Plant Physiol, 173, 1692-1708, 2017, DOI: org/10.1104/pp.16.01743).

So far, information on IQD proteins was sparse: With their eponymous IQ domain, IQDs Ca²⁺-dependently bind CaM. In Arabidopsis, a family of 33 IQD homologs has been identified. One family member from Arabidopsis, IQD1, plays a role in  defense mechanisms, another family member from tomato, SlIQD12, is involved in cell and fruit shape and size. Some Arabidopsis IQDs seem to associate with cytoskeletal proteins. But how does one comprehensively characterize a 33-membered protein family, whose main feature is a CaM-binding motif?

In their latest publication, Bürstenbinder et al. systematically study the subcellular localisation of the 33 Arabidopsis IQDs in order to obtain more hints regarding their function. With confocal laserscanning microscopy they found that most IQDs mainly localize to microtubuli. Microtubuli are part of the cytoskeleton, traversing the whole cell as fine, filamentous protein polymers, serving as routes of transportation and providing the cell with shape and physical stability. Together with bioinformatician Birgit Möller from the University of Halle, the researchers applied an elegant method to quantify microtubuli localisation of IQDs. Thereby, they could group IQDs according to their localisation pattern, and finally focus their following experiments on few representative IQDs. Interestingly, transgenic Arabidopsis plants with increased IQD levels showed twisted growth. Those plant lines, when observed under the microscope, exhibited elongated cells and microtubuli arrayed in skewed angles. This phenotype resembles mutants that are affected in microtubuli-associated processes. Thus, with their results, the researchers corroborate their hypothesis, that IQD proteins link CaM-mediated Ca²⁺ signalling with cell form, shape and function. One step further in understanding the numerous, specific outcomes of the universal cellular signal Calcium.