Species-Specific Functional Regions of the Green Alga Gamete Fusion Protein HAP2 Revealed by Structural Studies
39 Pages Posted: 29 Jan 2019 Sneak Peek Status: PublishedMore...
The cellular fusogen HAP2 drives gamete fusion across several eukaryotic kingdoms. It shares the 3-dimensional fold of viral class II fusion proteins. In contrast to fusion driven by viruses, cell-cell fusion is tightly controlled and allowed only at the right moment and with a same-species gamete. In spite of a conserved architecture, HAP2 displays highly species-specific functional regions not resolved in the available X-ray structure of the green alga Chlamydomonas reinhardtii HAP2 ectodomain. Here we present a new X-ray structure resolving these regions, showing a target-membrane interaction surface containing three amphipathic helices in a horseshoe-shaped arrangement. We also show that in green algae HAP2 features additional species-specific motifs, including a cystine-ladder-like module evocative of EGF-like motifs that make extracellular protein-protein interactions in animals. These motifs are inserted in loops critical for the fusogenic reorganization of class II proteins, suggesting potential species-specific interaction sites to control the gamete fusion reaction.
Suggested Citation: Suggested Citation