title: Structure-Function Studies Link Class II Viral Fusogens with the Ancestral Gamete Fusion Protein HAP2.
abstract: The conserved transmembrane protein, HAP2/GCS1, has been linked to fertility in a wide range of taxa and is hypothesized to be an ancient gamete fusogen. Using template-based structural homology modeling, we now show that the ectodomain of HAP2 orthologs from Tetrahymena thermophila and other species adopt a protein fold remarkably similar to the dengue virus E glycoprotein and related class II viral fusogens. To test the functional significance of this predicted structure, we developed a flow-cytometry-based assay that measures cytosolic exchange across the conjugation junction to rapidly probe the effects of HAP2 mutations in the Tetrahymena system. Using this assay, alterations to a region in and around a predicted "fusion loop" in T. thermophila HAP2 were found to abrogate membrane pore formation in mating cells. Consistent with this, a synthetic peptide corresponding to the HAP2 fusion loop was found to interact directly with model membranes in a variety of biophysical assays. These results raise interesting questions regarding the evolutionary relationships of class II membrane fusogens and harken back to a long-held argument that eukaryotic sex arose as the byproduct of selection for the horizontal transfer of a "selfish" genetic element from cell to cell via membrane fusion.
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Jennifer Fricke
authors.initials: JF
authors.lastname: Pinello
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Alex L
authors.initials: AL
authors.lastname: Lai
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Jean K
authors.initials: JK
authors.lastname: Millet
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Donna
authors.initials: D
authors.lastname: Cassidy-Hanley
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Jack H
authors.initials: JH
authors.lastname: Freed
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA. Electronic address: tgc3@cornell.edu.
authors.forename: Theodore G
authors.initials: TG
authors.lastname: Clark
mesh_terms: D005306:Fertilization; D005434:Flow Cytometry; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics*; D008958:Models, Molecular; D009154:Mutation; D017510:Protein Folding; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics*; D016808:Tetrahymena thermophila / Q000737:chemistry / Q000235:genetics / Q000502:physiology*
chemical_list: D008565:Membrane Proteins; D015800:Protozoan Proteins
keywords: GCS1; HAP2; Tetrahymena; conjugation; evolution of sex; membrane fusion; structure homology modeling; virus fusogen