Species-Specific Functional Regions of the Green Alga Gamete Fusion Protein HAP2 Revealed by Structural Studies

• Published in: Structure (London) Vol. 27; no. 1; pp. 113 - 124.e4
• Main Authors: Baquero, Eduard, Fedry, Juliette, Legrand, Pierre, Krey, Thomas, Rey, Felix A.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 02.01.2019; Elsevier (Cell Press); Cell Press
• Subjects: Amino Acid Motifs; Chlamydomonas reinhardtii; Chlamydomonas reinhardtii - chemistry; Chlamydomonas reinhardtii - metabolism; eukaryotic reproduction; fertilization; gamete fusion; Germ Cells; Germ Cells - physiology; green algae; last eukaryotic common ancestor; Life Sciences; membrane fusion; Membrane Proteins; Membrane Proteins - chemistry; Plant Proteins; Plant Proteins - chemistry; Protein Domains; protein evolution; structural biology; virus-cell gene exchanges in evolution; green algae; fertilization; protein evolution; Chlamydomonas reinhardtii; gamete fusion; last eukaryotic common ancestor; structural biology; virus-cell gene exchanges in evolution; eukaryotic reproduction; membrane fusion
• ISSN: 0969-2126; 1878-4186
• DOI: 10.1016/j.str.2018.09.014

The cellular fusion protein HAP2, which is structurally homologous to viral class II fusion proteins, drives gamete fusion across several eukaryotic kingdoms. Gamete fusion is a highly controlled process in eukaryotes, and is allowed only between same species gametes. In spite of a conserved architecture, HAP2 displays several species-specific functional regions that were not resolved in the available X-ray structure of the green alga Chlamydomonas reinhardtii HAP2 ectodomain. Here we present an X-ray structure resolving these regions, showing a target membrane interaction surface made by three amphipathic helices in a horseshoe-shaped arrangement. HAP2 from green algae also features additional species-specific motifs inserted in regions that in viral class II proteins are critical for the fusogenic conformational change. Such insertions include a cystine ladder-like module evocative of EGF-like motifs responsible for extracellular protein-protein interactions in animals, and a mucin-like region. These features suggest potential HAP2 interaction sites involved in gamete fusion control. [Display omitted] •Unprecedented organization of amphipathic α helices in the algal HAP2 fusion loops•An inserted EGF-like motif suggests a potential algal-specific fusion control site•An adjacent mucin-like region potentially modulates algal-specific interactions•Inter-chain stem/domain II interactions stabilize the post-fusion hairpin conformation Baquero et al. report the structure of the HAP2 gamete fusion protein of the Chlamydomonas reinhardtii alga, describing key elements required to bridge two opposite-type gametes and drive their fusion. Mutagenesis of these elements confirmed their membrane-interaction function. Additional specific HAP2 protein-protein interaction sites are described as potential fusion controls.