Assistant Research Scientist University of Maryland College Park College Park, Maryland
Fusion between two gametes is a defining event in the life of sexual eukaryotes, yet the mechanisms and molecules essential for gamete fusion are poorly characterized. During fertilization in the unicellular green alga, Chlamydomonasreinhardtii, two haploid gamete cells (one plus and one minus mating type) become activated by ciliary interactions with each other, shed their cell walls, and adhere and fuse their membranes at a specialized membrane protuberance known as the mating structure to create a single diploid zygote. Despite the diversity of fertilization strategies employed by green organisms, emerging evidence indicates a structural conservation among certain molecules driving their gamete membrane adhesion and fusion mechanisms. For instance, HAP2 is a class II membrane fusion protein on minus/male gametes required for gamete fusion in organisms across kingdoms. Here, we show that FUS1 on Chlamydomonas plus gametes and the previously identified plant sperm adhesion protein, GEX2, are members of a large, broadly conserved family of membrane adhesion proteins characterized by extracellular domains composed entirely of immunoglobulin (Ig)-like folds. We also discovered the FUS1 receptor protein on minus gametes, MinusAdhesion Receptor 1 (MAR1), and show that MAR1 binds directly to FUS1, that MAR1 localizes with HAP2 at the site of membrane fusion, and that MAR1 is essential for membrane adhesion and for gamete fusion. Notably, MAR1 is also functionally and biochemically associated with HAP2, and MAR1-dependent membrane adhesion triggers the HAP2 trimer formation that initiates gamete fusion in vivo. Thus, during fertilization in Chlamydomonas, the lineage-specific protein MAR1 functions at the nexus of the conserved membrane adhesion protein FUS1 and the ancient fusogen HAP2 to drive the merger of gamete membranes.