Perspective

The structural era of endocytosis that we have witnessed for the past decade or so has provided us with a detailed molecular framework of the mechanics of cargo selection, membrane deformation, and vesicle fission in the clathrin/AP-2 pathway. We have detailed knowledge of the elaborate network of protein-protein and pro-tein-lipid interactions within the endocytic mesh. AP-2, clathrin, and phosphoi-nositides appear to correspond to central hubs in the organization of the endocytic network. Based on the detailed biochemical and structural knowledge, we now seem to be in a position to use this information for the design of chemical approaches to rapidly and selectively interfere with clathrin/ AP-2 function in general or with cargo-selective pathways of internalization.

Automated screening paired with sensitive fluorescence-based functional assays in living cells and in vivo may pave the way to identify compounds that selectively target interaction hubs or specific interaction sites within a hub protein. We know now that intracellular signaling events are interrelated with receptor endocytosis. Moreover, gradients of morphogens are generated by fine-tuning of clathrin/AP-2-mediated endocytic events in complex cellular systems or developing polarized tissues (Berdnik et al. 2002; Dudu et al. 2004). The availability of selective pharmacological inhibitors targeting hubs within the endocytic network may provide important tools to acutely interfere with clathrin/AP-2-dependent endocytic mechanisms.

Perhaps, as importantly, the functional and molecular characterization of endocytic AP-2-binding accessory proteins as cargo-selective adaptors identifies these as potential targets for the design of selective inhibitors targeting subsets of clathrin-coated vesicles without perturbing endocytic mechanisms in general. If paired with powerful genetics including RNA interference-mediated knockdown of endocytic proteins, we are also in a position to ascertain that the pharmacological tools identified exhibit specific effects without affecting other cell physiological functions.

Acknowledgements Work in the authors' laboratory is supported by grants from the Deutsche Forschungsgemeinschaft (DFG; F0R806), the German Ministry of Science and Technology (BMBF, RENTRAFF), and the European Molecular Biology Organization (EMBO YIP award to V.H.).

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