FIGURE 4. Mechanisms by which endocytosis is controlled by phosphorylation.

A: phosphorylation of residues within or near internalization signals can enhance (AQP4) or inhibit (GABA-A receptor) their interaction with endocytotic machinery. B: in transport proteins, like the Na-K-ATPase (see text), phosphorylation at sites other than the endocytotic signal can create interaction sites for proteins that are indirectly involved in endocytosis. C: endocytotic machinery is also regulated by phosphorylation. For example, the binding site for tyrosine-based internalization signals on the AP-2 complex (mu subunit) is exposed on phosphorylation by AAK1 (or by PIP₂ association).