Either part of a Drosophila epsin protein, divided after the ENTH domain, functions in endocytosis of delta in the developing eye

Epsin is part of a protein complex that performs endocytosis in eukaryotes [1]. Drosophila epsin, Liquid facets (Lqf), was identified because it is essential for patterning the eye and other imaginal disc derivatives [2]. Previous work has provided only indirect evidence that Lqf is required for endocytosis in Drosophila [2, 3]. Epsins are modular and have an N-terminal ENTH (epsin N-terminal homology) domain that binds PIP2 at the cell membrane [4-6] and four different classes of protein-protein interaction motifs [1, 7-13]. The current model for epsin function in higher eukaryotes is that epsin bridges the cell membrane, a transmembrane protein to be internalized, and the core endocytic complex [14]. Here, we show directly that Drosophila epsin (Lqf) is required for endocytosis. Specifically, we find that Lqf is essential for internalization of the Delta (DI) transmembrane ligand in the developing eye. Using this endocytic defect in lqf mutants, we develop a transgene rescue assay and perform a structure/function analysis of Lqf. We find that when we divide Lqf into two pieces, an ENTH domain and an ENTH-less protein, each part retains significant ability to function in DI internalization and eye patterning. These results challenge the model for epsin function that requires an intact protein.