Abstract

At least two different surfaces of the AP-2 beta2 appendage domain can bind linear motifs in other endocytic regulatory proteins. The platform subdomain or top surface binds a helical [ED]x(1,2)Fxx[FL]xxxR motif found in Epsin-1 and -2 which bind ubiquitinated growth factor receptors, the beta-arrestins which bind GPCRs and ARH which binds LDL receptor family members. All of these function as cargo-selective clathrin adaptors, targeting surface receptors for internalization by clathrin-mediated endocytosis.

In beta-arrestin, the cargo motif is regulated by a remarkable structural rearrangement. The motif maintains the endocytosis-incompetent state by binding back on the folded core of the protein in a beta strand conformation. Apparently triggered via a beta-arrestin/GPCR interaction, the motif must be displaced and must undergo a strand to helix transition to enable the beta2 appendage binding step that drives GPCR-beta-arrestin complexes into clathrin coats.

The sandwich subdomain or side surface site binds an FxxxFxDF motif found in EPS15 and AP180 and may also accept other endocytosis proteins with variant motifs, as suggested by mutagenesis of the binding surface. The sandwich domain binders are accessory endocytic proteins (without a direct role in cargo binding) which help in CCV formation. Currently, there is no entry for the sandwich domain motif in ELM.