Transmembrane topology of mammalian planar cell polarity protein Vangl1

Vangl1 and Vangl2 are membrane proteins that play an important role in neurogenesis, and Vangl1/Vangl2 mutations cause neural tube defects in mice and humans. At the cellular level, Vangl proteins regulate the establishment of planar cell polarity (PCP), a process requiring membrane assembly of asymmetrically distributed multiprotein complexes that transmit polarity information to neighboring cells. The membrane topology of Vangl proteins and the protein segments required for structural and functional aspects of multiprotein membrane PCP complexes is unknown. We have used epitope tagging and immunofluorescence to establish the secondary structure of Vangl proteins, including the number, position, and polarity of transmembrane domains. Antigenic hemagglutinin A (HA) peptides (YYDVPDYS) were inserted in predicted intra- or extracellular loops of Vangl1 at positions 18, 64, 139, 178, 213, and 314, and individual mutant variants were stably expressed at the membrane of MDCK polarized cells. The membrane topology of the exofacial HA tag was determined by a combination of immunofluorescence in intact (extracellular epitopes) and permeabilized (intracellular epitopes) cells and by surface labeling. Results indicate that Vangl proteins have a four-transmembrane domain structure with the N-terminal portion (HA18 and HA64) and the large C-terminal portion (HA314) of the protein being intracellular. Topology mapping and hydropathy profiling show that the loop delineated by TMD1-2 (HA139) and TMD3-4 (HA213) is extracellular while the segment separating predicted TMD2-3 (HA178) is intracellular. This secondary structure reveals a compact membrane-associated portion with very short predicted extra- and intracellular loops, while the protein harbors a large intracellular domain.