Phosphoinositides and PIPs

Phosphatidylinositols or PI are membrane-bound lipids that are part of signaling pathways that regulate membrane traffic. They can be phosphorylated  at the carbon-3, carbon-4 and carbon 5 - positions of the inositol sugar rings to produce different phosphoinositides or PIPs.

Each organelle has its own set of enzymes to catalyze the rapid interconversion of PI and PIPs through phosphorylation and dephosphorylation. Through such modifications, these enzymes locally control the rapid binding of proteins to a membrane.

The distribution of PIPs varies between organelles as well as between regions of a continuous membrane, defining specialized membrane domains that impart a unique surface identity.

Each particular type of PIP recruits specific proteins that attach their head groups through their PIP-binding domains. The binding of proteins is locally controlled by enzymes such as PI Kinase , PIP kinase , and PIP phosphatase.

For example, PI Kinase converts Phosphatidylinositol 4-phosphate to Phosphatidylinositol 4,5-bisphosphate, which can bind Adaptor Protein AP2 on the cytosolic face of the plasma membrane. This interaction changes the conformation of the PIP, allowing the membrane to bend and expose the binding sites of embedded cargo receptors.

Once the soluble cargo molecules are loaded, the vesicles formed are ready to bud out from the plasma membrane.