Mechanisms and Consequences of Inactivating the AP2 Clathrin Adaptor Complex

Abstract

Eukaryotic cells internalize cell-surface transmembrane proteins through the fundamental process called clathrin-mediated endocytosis (CME). How CME occurs with spatiotemporal precision is still unclear. Because the Adaptor Protein 2 (AP2) complex initiates endocytic events, modulating AP2 is one way to control CME. In this work, we discovered that AP2 inactivation is a regulated process. We combined C. elegans genetics with in vitro biochemistry and live microscopy and found that phosphorylation of AP2 marks the complex for subsequent inactivation by the protein called NECAP. In the second part of my work, I explored the biology underlying an intriguing skin-specific pathology in C. elegans. This pathology is called ‘jowls’ and is caused by mutations that inactivate AP2. We identified novel, gain-of-function mutations in MLT-4, a homolog of Inversin, that also cause jowls. In a genetic screen for suppressors of the MLT-4 jowls alleles, we isolated a novel regulator of the pathology: APE-1, the C. elegans homolog of the Ankyrin repeat, SH3 domain, Proline-rich-region-containing Proteins (ASPPs). ASPPs are a conserved family of Protein Phosphatase 1 catalytic subunit (PP1c) binding partners that localize to intercellular junctions and are associated with cardiocutaneous diseases. Prior to this work, how ASPPs localize to junctions and whether they modulate PP1c activity in vivo remained unclear. Using structure-function analysis, live microscopy, and an in vivo functional assay, we discovered that sequence in APE-1’s N-terminal region directs the APE-1–PP1c complex to junctions, while APE-1’s C-terminal domains modulate PP1c output. Together, my work has uncovered how the AP2 clathrin adaptor complex is negatively regulated and characterized biology that is perturbed in C. elegans when AP2 is inactive.