The handle
http://hdl.handle.net/1887/71939
holds various files of this Leiden University
dissertation.
Author: Habets, M.E.J.
Title: Regulation of the Arabidopsis AGC kinase PINOID by PDK1 and the microtubule
cytoskeleton
SIX
SUMMARY
185
During their life cycle, organisms react to various external signals in order to avoid adverse environmental conditions. The ways various organisms react to these signals are diverse. Animals, for example, are generally mobile and therefore have the possibility to move to a different location to improve their living conditions. Plants, in contrast, are sessile organisms, and therefore have developed alternative strategies to adapt to changes in their environment, in part by altering their growth and development. When a signal is percieved by a receptor protein, the signal is generally transduced via one or multiple chemical or physical intermediates, which eventually results in a cellular response. During this signal transduction cells can utilize various processes including, but not limited to, protein phosphorylation, changes in membrane lipid composition, changes in metabolism and gene expression. The final result of these changes is the response that puts the cells, in case of multicellular organisms the tissues and the organism as a whole in a state to handle the signal. One of the central regulators of this adaptive plant growth and development is the plant hormone auxin or indole-3-acetic acid (IAA). Auxin is involved in directional growth responses of plant roots and shoots to signals; such as gravity, light and mechanical stress, and it also positions and regulates the outgrowth of new organs.
GNOM it is well established that it is responsible for the PIN polarity at the rootward (basal) PM domain. Moreover, phosphorylation of PIN proteins by the plant-specific protein kinase A, cyclic GMP-dependent protein kinase, protein kinase C (AGC) kinases PINOID (PID), WAG1 and WAG2 in arabidopsis leads to their sorting to the GNOM-independent shootward (apical) sorting pathway.
Beside the transport of auxin, cells also need to sense the hormone quantitatively in order to respond to it. In chapter 2 we discuss our view on a recent finding in this auxin perception. The Auxin Binding Protein 1 (ABP1) is encoded by a single copy gene in Arabidopsis thaliana (arabidopsis), and since its discovery the consensus was that abp1 loss-of-function results in embryo lethality. A recent paper describes the creation of a new loss-of-function line that was found to be not embryo lethal, putting its status as one of the auxin receptors in jeopardy. We provide an overview of all alleles and constructs that have been used in the past 40 years of ABP1 research and give possible explanations for the observed conflicting results between the recently published paper and previous research.
187 regulators of the PID kinase we identified arabidopsis BTB and TAZ domain protein1 (BT1) as a PID binding protein. The BT1 gene belongs to a five-member gene family in arabidopsis, encoding proteins with a land plant-specific domain structure consisting of an amino-terminal BTB domain, a TAZ domain and a carboxy-terminal calmodulin binding domain. At least four of the five BT proteins interacted with PID through their BTB domain, and in vitro phosphorylation assays indicated that BT1 is not a phosphorylation target of PID, but that BT1 binding reduces the activity of the kinase. BT1 localized in the nucleus and the cytoplasm. Upon co-expression with PID, BT1 was found at the plasma membrane, whereas PID localization became partially nuclear. Overexpression of BT1 led to a reduction of PID overexpression seedling phenotypes and enhanced pid loss-of-function embryo phenotypes. In contrast, bt loss-of-function enhanced adult phenotypes of PID overexpression plants. A subsequent yeast two-hybrid screen for BT1 interacting proteins yielded two At1-family kinesins (which we named PBK1 and PBK2) that were found to induce BT1-dependent relocalization of PID and its closest family members WAG1, WAG2 and AGC3-4 to the MT in arabidopsis protoplasts. Together these data suggest that BT1 acts as signaling scaffold that regulates AGC3 kinase activity in part by relocating PID to the nucleus or, for all the kinases, to the MT.
kinesins, it is tempting to speculate that the BT1-PBK complex is involved in relocating PID to the phragmoplast during cell division.
