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

Eduard Baquero,Juliette Fedry,Pierre Legrand,Thomas Krey,Felix A Rey

doi:10.1016/j.str.2018.09.014

Abstract

SummaryThe 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.

Highlights

We recently reported the X-ray structure of the C. reinhardtii HAP2 ectodomain
We screened for crystallization conditions of the unproteolyzed HAP2 ectodomain in complex with HAP2-specific monoclonal antibodies obtained previously (Fedry et al, 2017)
Structure determination by molecular replacement using the atomic model of HAP2, PDB: 5MF1, resulted in an electron density map of HAP2 displaying one protomer per asymmetric unit and 80% of the volume occupied by solvent (Table 1)

Summary

Introduction

It is required for fertilization, where two gametes of opposite type merge together to form a zygote (Georgadaki et al, 2016). The only well-characterized extracellular fusion process is that of enveloped viruses, which fuse the viral lipid bilayer either with the plasma membrane or with an endosome from the luminal side (Harrison, 2015). The gamete fusion process has been studied in highest detail in C. reinhardtii, where HAP2 was shown to localize to the ‘‘mating structure’’, a projecting organelle at the plasma membrane located in between the two flagella of the minus gametes (Pan et al, 2003)

Methods

Results

Discussion

Conclusion