Conformational regulation of AP1 and AP2 clathrin adaptor complexes

Abstract

Heterotetrameric clathrin adaptor protein complexes (APs) orchestrate the formation of coated vesicles for transport among organelles of the cell periphery. AP1 binds membranes enriched for phosphatidylinositol 4‐phosphate, such as the trans Golgi network, while AP2 associates with phosphatidylinositol 4,5‐bisphosphate of the plasma membrane. At their respective membranes, AP1 and AP2 bind the cytoplasmic tails of transmembrane protein cargo and clathrin triskelions, thereby coupling cargo recruitment to coat polymerization. Structural, biochemical and genetic studies have revealed that APs undergo conformational rearrangements and reversible phosphorylation to cycle between different activity states. While membrane, cargo and clathrin have been demonstrated to promote AP activation, growing evidence supports that membrane‐associated proteins such as Arf1 and FCHo also stimulate this transition. APs may be returned to the inactive state via a regulated process involving phosphorylation and a protein called NECAP. Finally, because antiviral mechanisms often rely on appropriate trafficking of membrane proteins, viruses have evolved novel strategies to evade host defenses by influencing the conformation of APs. This review will cover recent advances in our understanding of the molecular inputs that stimulate AP1 and AP2 to adopt structurally and functionally distinct configurations.

Control of clathrin-mediated endocytosis by NIMA family kinases

“…The F-BAR domain binds to lipid bilayers and aids in membrane curvature, whereas the Mu-homology domain binds to cargo and other endocytic adapter proteins. The AP2 Activator domain has recently been shown to facilitate the allosteric opening/activation of AP2 (Fig 2B), and loss of fcho-1 activity in C. elegans leads to a decrease in the levels of open/active AP2 [67-69].…”

“…Recently, C. elegans NCAP-1 was shown to allosterically regulate AP2 to promote the closed/inactive conformation, and loss of ncap-1 suppresses the slow-growth phenotype of fcho-1 strains [70]. Thus, NCAP-1 acts in opposition to FCHO-1 to regulate AP2 conformation and activity (Fig 3A) [67].…”

“…Chief among these is the conserved plasma membrane adapter protein complex, AP2 [57, 61-63]. AP2 consists of four subunits, termed α, β, µ, and σ, and exists in at least two functionally distinct structures, broadly termed the open/active and closed/inactive conformations [55, 57, 64-67]. Allosteric regulators of AP2 conformation include FCHo1/2, which promotes the open state of AP2 [68, 69], and NECAP1/2, which promotes the closed conformation [70].…”

“…Allosteric regulators of AP2 conformation include FCHo1/2, which promotes the open state of AP2 [68, 69], and NECAP1/2, which promotes the closed conformation [70]. In addition, protein kinases have been implicated in the regulation of AP2 through phosphorylation of the µ subunit [67, 71].…”

Overview: Traffic at atomic resolution

“…Xie and Ren highlight newly determined X‐ray crystal structures that uncover mechanisms for exporting mRNA from the nucleus to cytoplasm. Two reviews highlight advances in understanding familiar coat proteins: Audhya co‐workers describe new regulatory factors in COPII trafficking, while Hollopeter and co‐workers highlight advances in understanding conformational regulatory changes by the AP1 and AP2 clathrin adaptors. Advances in single particle cryo‐EM enable mechanistic insights into dynamin‐related proteins from Ford and Chappie and into tethers from Ungermann and Kümmel .…”