Arabidopsis thaliana ROP GTPase activating proteins: At the crossroad of cell division and cell expansion

Abstract

Cell division is a fundamental process in every living organism. Therefore it is important to investigate this mechanism. Higher plants developed a separate kind of cell division since the cells are not motile unlike mammal or yeast cells. Land plants have to overcome the rigidity of the cellulosic cell wall. Plants developed specific cytoskeletal arrays, the preprophase band and the phragmoplast. The preprophase band predicts the insertion site of the future, partitioning new wall, the cell plate. The cell plate is formed by vesicle fusion, initiated in the center of the cell and expands towards the parental plasma membrane, where the preprophase band originally predicted the insertion sites. The cell plate formation is facilitated by the plant specific cytoskeletal array, the phragmoplast. Since the preprophase band disappears at the end of prophase it is important to maintain the positional information it provides. In the model plant Arabidopsis thaliana (A. thaliana) two members of the kinesin-12 class motor proteins, PHRAGMOPLAST ORIENTING KINESIN (POK) 1 and 2 preserve this information while localizing at the former position of the preprophase band throughout mitosis and cytokinesis. The absence of these two proteins results in mis-positioned cell walls indicating that POK1 and POK2 carry out an important function in cell wall positioning during cell division. For better understanding of POK function and its role in plant cell division, it is essential to have a closer look at the network of interacting proteins. So far, TANGLED and RANGAP1 are known as interaction partners of POK1 and present the identity of the CDZ. In this study two novel interaction partners of POK1 were characterized and investigated in the context of cell division and cell shape establishment. Both identified proteins are Rho of plant (ROP)-GTPase activating proteins (GAPs), designated PHGAP1 and PHGAP2. They belong to a small subfamily of Rho of plant GTPase activating proteins (ROPGAPs) characterized by a Pleckstrin Homology (PH) domain. Both PHGAP1 and PHGAP2, co-localize with POK1 at the cortical division site. The phgap1 phgap2 double mutant embryo analysis suggests that both proteins are functionally redundant and contribute to the correct insertion of the new cell wall. In addition to their function in cell division, they play a major role in pavement cell establishment. phgap1 phgap2 knockdown mutants display a severely altered pavement cell shape suggesting that regulation of ROPs, by PHGAP, is important for polar pavement cell growth, which is achieved by inducing actin filament network or microtubule bundling. This study suggests that PHGAP1 and PHGAP2 regulate the cytoskeleton by inactivating certain ROPs during different developmental stages.