NT-C2-Dependent Phosphoinositide Binding Controls PLASTID MOVEMENT IMPAIRED1 Localization and Function

Plants respond to changing environmental conditions through rapid cellular mechanisms, one of which is light-induced chloroplast movement. Plastid Movement Impaired 1 (PMI1) is one of the proteins involved in this process that undergoes rapid, blue light-dependent relocalization within the plasma membrane. Here, we investigate the molecular determinants of PMI1 membrane association. We identify the NT-C2 domain as the principal membrane-binding module and show that it extends beyond the boundaries previously assigned to the C2 domain. Plasma membrane localization is mediated by interactions between the extended NT-C2 domain and the phosphoinositides phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], with basic residues within this region being essential for PMI1 membrane binding. We further demonstrate that the NT-C2 domain binds Ca2+ in vitro and that calcium availability modulates its phosphoinositide-binding preference. Consistently, depletion of cytosolic Ca2+ or inhibition of Ca2+ fluxes abolished the blue light-induced redistribution of PMI1 within the plasma membrane. Finally, we show that the extended NT-C2 domain, together with its flanking intrinsically disordered regions, is indispensable for PMI1 function in chloroplast movement regulation.