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Phosphoinositide lipids and cell polarity: linking the plasma membrane to the cytocortex.

Many cell types in animals and plants are polarized, which means that the cell is subdivided into functionally and structurally distinct compartments. Epithelial cells, for example, possess an apical side facing a lumen or the outside environment and a basolateral side facing adjacent epithelial cells and the basement membrane. Neurons possess distinct axonal and dendritic compartments with specific functions in sending and receiving signals. Migrating cells form a leading edge that actively engages in pathfinding and cell-substrate attachment, and a trailing edge where such attachments are abandoned. In all of these cases, both the plasma membrane and the cytocortex directly underneath the plasma membrane show differences in their molecular composition and structural organization. In this chapter we will focus on a specific type of membrane lipids, the phosphoinositides, because in polarized cells they show a polarized distribution in the plasma membrane. They furthermore influence the molecular organization of the cytocortex by recruiting specific protein binding partners which are involved in the regulation of the cytoskeleton and in signal transduction cascades that control polarity, growth and cell migration.