Structure-Function Studies Link Class II Viral Fusogens with the Ancestral Gamete Fusion Protein HAP2

• Published in: Current biology Vol. 27; no. 5; pp. 651 - 660
• Main Authors: Pinello, Jennifer Fricke, Lai, Alex L., Millet, Jean K., Cassidy-Hanley, Donna, Freed, Jack H., Clark, Theodore G.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 06-03-2017; Elsevier
• Subjects: conjugation; evolution of sex; Fertilization; Flow Cytometry; GCS1; HAP2; Life Sciences; membrane fusion; Membrane Proteins - chemistry; Membrane Proteins - genetics; Models, Molecular; Mutation; Protein Folding; Protozoan Proteins - chemistry; Protozoan Proteins - genetics; structure homology modeling; Tetrahymena; Tetrahymena thermophila - chemistry; Tetrahymena thermophila - genetics; Tetrahymena thermophila - physiology; virus fusogen; Tetrahymena; structure homology modeling; HAP2; virus fusogen; GCS1; evolution of sex; conjugation; membrane fusion
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2017.01.049

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.
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•HAP2/GCS1 and class II viral fusogens adopt a remarkably similar protein fold•HAP2 mediates membrane pore formation in mating Tetrahymena cells•The T. thermophila HAP2 ortholog has a functional fusion loop peptide•HAP2 may have arisen from a virus and been key to the origin of eukaryotic sex
HAP2/GCS1 is deeply rooted in evolution and required for gamete fusion in diverse species. Pinello et al. demonstrate that a HAP2 ortholog from Tetrahymena closely resembles class II viral fusogens. From an evolutionary standpoint, it is unclear which came first, but HAP2’s emergence may have been a critical step in the evolution of eukaryotic sex.