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Vol. 19, Issue 12, 5309-5326, December 2008

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The AP-2 Adaptor β2 Appendage Scaffolds Alternate Cargo Endocytosis

Peter A. Keyel^*,,, James R. Thieman^*,, Robyn Roth, Elif Erkan^||, Eric T. Everett^¶, Simon C. Watkins^*, John E. Heuser, and Linton M. Traub^*

*Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261; Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO 63110; ^||Children's Hospital of Pittsburgh, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213; and ^¶Department of Pediatric Dentistry and The Carolina Center for Genome Science, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599

Submitted July 11, 2008; Revised September 19, 2008; Accepted September 30, 2008
Monitoring Editor: Sandra Lemmon

The independently folded appendages of the large and β2 subunits of the endocytic adaptor protein (AP)-2 complex coordinate proper assembly and operation of endocytic components during clathrin-mediated endocytosis. The β2 subunit appendage contains a common binding site for β-arrestin or the autosomal recessive hypercholesterolemia (ARH) protein. To determine the importance of this interaction surface in living cells, we used small interfering RNA-based gene silencing. The effect of extinguishing β2 subunit expression on the internalization of transferrin is considerably weaker than an AP-2 subunit knockdown. We show the mild sorting defect is due to fortuitous substitution of the β2 chain with the closely related endogenous β1 subunit of the AP-1 adaptor complex. Simultaneous silencing of both β1 and β2 subunit transcripts recapitulates the strong subunit RNA interference (RNAi) phenotype and results in loss of ARH from endocytic clathrin coats. An RNAi-insensitive β2-yellow fluorescent protein (YFP) expressed in the β1 + β2-silenced background restores cellular AP-2 levels, robust transferrin internalization, and ARH colocalization with cell surface clathrin. The importance of the β appendage platform subdomain over clathrin for precise deposition of ARH at clathrin assembly zones is revealed by a β2-YFP with a disrupted ARH binding interface, which does not restore ARH colocalization with clathrin. We also show a β-arrestin 1 mutant, which engages coated structures in the absence of any G protein-coupled receptor stimulation, colocalizes with β2-YFP and clathrin even in the absence of an operational clathrin binding sequence. These findings argue against ARH and β-arrestin binding to a site upon the β2 appendage platform that is later obstructed by polymerized clathrin. We conclude that ARH and β-arrestin depend on a privileged β2 appendage site for proper cargo recruitment to clathrin bud sites.

These authors contributed equally to this work.

Present address: Department of Medicine and Howard Hughes Medical Institute, Washington University School of Medicine, St. Louis, MO 63110.

Address correspondence to: Linton M. Traub (traub{at}pitt.edu)

Abbreviations used: ARH, autosomal recessive hypercholesterolemia protein; CLASP, clathrin-associated sorting protein; Dab2, disabled-2; EGFP, enhanced green fluorescent protein; GPCR, G protein-coupled receptor; HC, heavy chain; LDL, low-density lipoprotein; mAb, monoclonal antibody; MPR, cation-independent mannose 6-phosphate receptor; PtdIns(4,5)P₂, phosphatidylinositol 4,5-bisphosphate; PTB, phosphotyrosine binding; tdRFP, tandem dimer tomato; Tf488, transferrin conjugated to Alexa Fluor 488; Tf568, transferrin conjugated to Alexa Fluor 568; Tf633, transferrin conjugated to Alexa Fluor 633; YFP, yellow fluorescent protein.

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