Endocytosis assays in intact and permeabilized cells

Numb regulates vesicular docking for homotypic fusion of early endosomes via membrane recruitment of Mon1b

Numb is an endocytic protein that plays crucial roles in diverse cellular processes such as asymmetric cell division, cell migration and differentiation. However, the molecular mechanism by which Numb regulates endocytic trafficking is poorly understood. Here, we demonstrate that Numb is a docking regulator for homotypic fusion of early endosomes (EEs). Numb depletion causes clustered but unfused EEs, which can be rescued by overexpressing cytosolic Numb 65 and Numb 71 but not plasma membrane-attached Numb 66 or Numb 72. Time-lapse analysis reveals that paired vesicles tend to tether but not fuse with each other in the absence of Numb. We further show that Numb binds to another docking regulator, Mon1b, and is required for the recruitment of cytosolic Mon1b to the EE membrane. Consistent with this, deletion of Mon1b causes similar defects in EE fusion. Our study thus identifies a novel mechanism by which Numb regulates endocytic sorting by mediating EE fusion.

Systematic analysis of endocytosis by cellular perturbations

Endocytosis is an essential process of eukaryotic cells that facilitates numerous cellular and organismal functions. The formation of vesicles from the plasma membrane serves the internalization of ligands and receptors and leads to their degradation or recycling. A number of distinct mechanisms have been described over the years, several of which are only partially characterized in terms of mechanism and function. These are often referred to as novel endocytic pathways. The pathways differ in their mode of uptake and in their intracellular destination. Here, an overview of the set of cellular proteins that facilitate the different pathways is provided. Further, the approaches to distinguish between the pathways by different modes of perturbation are critically discussed, emphasizing the use of genetic tools such as dominant negative mutant proteins.

WASH and the Arp2/3 complex regulate endosome shape and trafficking

Activators of the Arp2/3 complex, termed nucleation-promoting factors (NPFs), are required for the proper spatial and temporal control of actin assembly in cells. Mammalian cells express several NPFs, each of which functions in a distinct cellular process, including WASP and N-WASP in phagocytosis and endocytosis, WAVE and JMY in cell migration, and WHAMM in ER-to-Golgi transport. Although another NPF called WASH was recently identified, the cellular localization and function of this protein were unclear. Here we demonstrated that human WASH alone potently activated the Arp2/3 complex in vitro and in cells, suggesting that the protein is not autoinhibited like N-WASP, but is likely regulated by interacting proteins. In cells, WASH was associated with Rab5-positive early endosomes and Rab11-positive recycling endosomes that were enriched for actin filaments. Silencing of WASH or Arp2/3 complex expression by RNAi, or disruption of actin function by drug treatments, caused enlargement and elongation of endosomes. Intriguingly, WASH silencing, as well as actin disruption, delayed EGF transport to LAMP1-positive late endosomes. These observations indicate that actin polymerization by WASH influences the shape and maturation of endosomes, and highlight a previously unrecognized role for WASH and the Arp2/3 complex in the degradative steps of endocytic trafficking.