Endocytosis is required for internalization of micronutrients and turnover of membrane components. Endophilin has been assigned as a component of clathrin-mediated endocytosis. Here we show in mammalian cells that endophilin marks and controls a fast-acting tubulovesicular endocytic pathway that is independent of AP2 and clathrin, activated upon ligand binding to cargo receptors, inhibited by inhibitors of dynamin, Rac, phosphatidylinositol-3-OH kinase, PAK1 and actin polymerization, and activated upon Cdc42 inhibition. This pathway is prominent at the leading edges of cells where phosphatidylinositol-3,4-bisphosphate-produced by the dephosphorylation of phosphatidylinositol-3,4,5-triphosphate by SHIP1 and SHIP2-recruits lamellipodin, which in turn engages endophilin. This pathway mediates the ligand-triggered uptake of several G-protein-coupled receptors such as α2a- and β1-adrenergic, dopaminergic D3 and D4 receptors and muscarinic acetylcholine receptor 4, the receptor tyrosine kinases EGFR, HGFR, VEGFR, PDGFR, NGFR and IGF1R, as well as interleukin-2 receptor. We call this new endocytic route fast endophilin-mediated endocytosis (FEME).

Clathrin-independent endocytosis (CIE) mediates the cellular uptake of many extracellular ligands, receptors, and pathogens, including several life-threatening bacterial toxins and viruses. So far, our understanding of CIE carrier formation has lagged behind that of clathrin-coated vesicles. Impediments have been the imprecise definition of some CIE pathways, the lack of specific cargoes being transported and of exclusive cytosolic markers and regulators. Notwithstanding these limitations, three distinct molecular mechanisms by which CIE carriers form can be defined. Cargo capture by cytosolic proteins is the main mechanism used by fast endophilin-mediated endocytosis (FEME) and interleukin 2 receptor (IL-2R) endocytosis. Acute signaling-induced membrane remodeling drives macropinocytosis. Finally, extracellular lipid or cargo clustering by the glycolipid-lectin (GL-Lect) hypothesis mediates the uptake of Shiga and cholera toxins and receptors by the CLIC/GEEC pathway. Here, we review these mechanisms and highlight current gaps in knowledge that will need to be addressed to complete our understanding of CIE.