REGULATORY INTERACTIONS IN THE RECOGNITION OF ENDOCYTIC SORTING SIGNALS BY AP-2 COMPLEXES

Many plasma membrane proteins destined for endocytosis are concentrate d into clathrin-coated pits through the recognition of a tyrosine-base d moth in their cytosolic domains by an adaptor (AP-2) complex. The mu 2 subunit of isolated AP-2 complexes binds specifically, but rather w eakly, to proteins bearing the tyrosine-based signal. We now demonstra te, using peptides with a photoreactive probe, that this binding is st rengthened significantly when the AP-2 complex is present in clathrin coats, indicating that there is cooperativity between receptor-AP-2 in teractions and coat formation. Phosphoinositides with a phosphate at t he D-3 position of the inositol ring, but not other isomers, also incr ease the affinity of the AP-2 complex for the tyrosine-based moth. AP- 2 is the first protein known (in any context) to interact with phospha tidyl-inositol 3-phosphate. Our findings indicate that receptor recrui tment can be coupled to clathrin coat assembly and suggest a mechanism for regulation of membrane traffic by lipid products of phosphoinosit ide 3-kinases.