EHD proteins: key conductors of endocytic transport

Abstract

Regulation of endocytic transport is controlled by an elaborate network of proteins. Rab GTP-binding proteins and their effectors have well-defined roles in mediating specific endocytic transport steps, but until recently less was known about the four mammalian dynamin-like C-terminal Eps15 homology domain (EHD) proteins that also regulate endocytic events. In recent years, however, great strides have been made in understanding the structure and function of these unique proteins. Indeed, a growing body of literature addresses EHD protein structure, interactions with binding partners, functions in mammalian cells, and the generation of various new model systems. Accordingly, this is now an opportune time to pause and review the function and mechanisms of action of EHD proteins, and to highlight some of the challenges and future directions for the field.

Figure 1

Domain architecture, conservation and function of C-terminal Eps15 Homology Domain (EHD) proteins. (A) The EHD proteins, comprised of 534–543 amino acids, each contain two helical regions, a conserved ATP-binding domain, a linker region and an EH domain localized to the N-terminus of the protein. (B) Comparison of the amino acid sequence identity of full-length EHD proteins and their individual EH-domains. (C) Partial list of characterized EHD1 and EHD2 mutants and their functions or phenotypes. (D) Proposed model of EHD protein function. Cytoplasmic localized EHD proteins bind ATP and dimerise. EHD dimerisation causes the formation of a membrane binding site and the EHD proteins associate with tubular membranes, where they undergo further oligomerisation. Upon ATP hydrolysis, the membranes are destabilised, leading to scission of vesicles containing concentrated cargo/receptors, thus facilitating vesicular transport.

Figure 2

Regulation of endocytic transport by EHD proteins, Rabs and their effectors. Internalised receptors reach the sorting or early endosome (EE) and are trafficked through one of at least four pathways. Receptors slated for degradation are sorted to EE microdomains containing EHD4, Rab5 and Rabenosyn-5 and transported to late endosomes and lysosomes. Other internalised proteins, such as the Shiga toxin, are transported from EE to the Golgi via an EHD3 and/or EHD1-dependent retrograde pathway. Receptors that recycle back to the plasma membrane can do so directly from EE in a poorly defined manner that requires the function of Rab4 (Fast recycling). Alternatively, many receptors are first directed to the perinuclear endocytic recycling compartment and then shuttled to the plasma membrane (Slow recycling). Slow recycling requires the sequential function of multiple regulators including Rab5, Rab11, Rab8a their effectors (Rabenosyn-5, Rab11-FIP2 and MICAL-L1, respectively), and EHD proteins.