Completing the family of human Eps15 homology domains: Solution structure of the internal Eps15 homology domain of gamma-synergin

Eps15 homology (EH) domains are universal interaction domains to establish networks of protein-protein interactions in the cell. These networks mainly coordinate cellular functions including endocytosis, actin remodeling, and other intracellular signaling pathways. They are well characterized in structural terms, except for the internal EH domain from human gamma-synergin (EH gamma). Here, we complete the family of EH domain structures by determining the solution structure of the EH gamma domain. The structural ensemble follows the canonical EH domain fold and the identified binding site is similar to other known EH domains. But EH gamma differs significantly in the N- and C-terminal regions. The N-terminal alpha-helix is shortened compared to known homologues, while the C-terminal one is fully formed. A significant proportion of the remaining N- and C-terminal regions are well structured, a feature not seen in other EH domains. Single mutations in both the N-terminal and the C-terminal structured extensions lead to the loss of the distinct three-dimensional fold and turn EH gamma into a molten globule like state. Therefore, we propose that the structural extensions in EH gamma function as a clamp and are undoubtedly required to maintain its tertiary fold.