Cargo Recognition in Clathrin-Mediated Endocytosis

Abstract

The endosomal system is expansive and complex, characterized by swift morphological transitions, dynamic remodeling of membrane constituents, and intracellular positioning changes. To properly navigate this ever-altering membrane labyrinth, transmembrane protein cargoes typically require specific sorting signals that are decoded by components of protein coats. The best-characterized sorting process within the endosomal system is the rapid internalization of select transmembrane proteins within clathrin-coated vesicles. Endocytic signals consist of linear motifs, conformational determinants, or covalent modifications in the cytosolic domains of transmembrane cargo. These signals are interpreted by a diverse set of clathrin-associated sorting proteins (CLASPs) that translocate from the cytosol to the inner face of the plasma membrane. Signal recognition by CLASPs is highly cooperative, involving additional interactions with phospholipids, Arf GTPases, other CLASPs, and clathrin, and is regulated by large conformational changes and covalent modifications. Related sorting events occur at other endosomal sorting stations.

Clathrin Terminal Domain-Ligand Interactions Regulate Sorting of Mannose 6-Phosphate Receptors Mediated by AP-1 and GGA Adaptors

“…At the plasma membrane, clathrin-mediated endocytosis (CME) regulates the cell surface levels and internalization of important plasma membrane proteins including receptors, transporters, ion channels, and synaptic vesicle proteins in brain (1)(2)(3)(4)(5). CME is also hijacked by pathogens such as HIV or bacteria (i.e., Listeria) to gain access to the cell interior.…”

“…Assembly of endocytic clathrin-coated pits (CCPs) is initiated by the recruitment of early endocytic adaptors such as AP-2 and FCHo 1/2 to plasma membrane sites enriched in phosphatidylinositol 4,5-bisphosphate. Together with other endocytic proteins, these factors couple clathrin recruitment and assembly to the selection of transmembrane cargo and the remodeling of the plasma membrane to eventually generate endocytic vesicles destined for endosomal fusion (3)(4)(5). In addition to CME, eukaryotic cells also express several clathrin-independent endocytic pathways (6).…”

Molecular Structure, Function, and Dynamics of Clathrin-Mediated Membrane Traffic

“…A large number of other receptors transit the same route. Sequences ("sorting signals") in the receptor cytoplasmic tails determine inclusion, and various adaptors, especially the heterotetrameric adaptor complex AP2, bind specifically to these signal segments while also associating with clathrin (Ohno et al 1995; see also Traub and Bonifacino 2013). Endocytic membrane reuptake after neurotransmitter release from synaptic vesicles appears to follow a closely related mechanism.…”

“…Clathrin coat assembly at the plasma membrane requires both recruitment of clathrin by lipid-associated adaptor proteins and cooperativity of lattice interactions. The nonstructural proteins, collectively termed endocytic accessory proteins, ensure efficient coated-pit assembly, cargo recruitment, and prompt coat disassembly after budding (see Traub and Bonifacino 2013;Merrifield and Kaksonen 2014).…”

“…The heterotetrameric adaptors link the clathrin coat and the membrane bilayer, and they are the principal cargo-recognition molecules. There are also several more-specialized adaptor proteins, which recognize particular receptors; the specialized adaptors often interact with both clathrin and with the heterotetrameric adaptors, increasing the repertoire of cargo that can be sorted (see Traub and Bonifacino 2013). Additional abundant proteins include components of the uncoating machinery (auxilin-1 and -2 and Hsc70), AP180/CALM, a molecule that recruits VAMP2,7 and 8, RME-6, a Rab5 GEF, and HIP1R (huntingtin-interacting-protein-1 related), an actin regulator that associates with clathrin light chains.…”