Abstract
Recognition and fusion between gametes during fertilization is an ancient process. Protein HAP2, recognized as the primordial eukaryotic gamete fusogen, is a structural homolog of viral class II fusion proteins. The mechanisms that regulate HAP2 function, and whether virus-fusion-like conformational changes are involved, however, have not been investigated. We report here that fusion between plus and minus gametes of the green alga Chlamydomonas indeed requires an obligate conformational rearrangement of HAP2 on minus gametes from a labile, prefusion form into the stable homotrimers observed in structural studies. Activation of HAP2 to undergo its fusogenic conformational change occurs only upon species-specific adhesion between the two gamete membranes. Following a molecular mechanism akin to fusion of enveloped viruses, the membrane insertion capacity of the fusion loop is required to couple formation of trimers to gamete fusion. Thus, species-specific membrane attachment is the gateway to fusion-driving HAP2 rearrangement into stable trimers.
Highlights
Recognition and fusion between gametes during fertilization is an ancient process
Even though gametes expressing a HAP2 mutant lacking hydrophobic residues in its fusion loop are incapable of fusion with WT plus gametes, HAP2 still re-organizes into trimers, indicating that FUS1-based interactions between the membranes activate HAP2 for fusion by converting it into a form that can transit to its postfusion form independently of target membrane insertion
The gamete membrane fusion reaction is driven by a fusogenic conformational change of HAP2 into trimers, the membrane interaction capacity of the fusion loop is required to couple trimer formation to bilayer merger, and species-specific membrane attachment catalyzes the transition
Summary
Recognition and fusion between gametes during fertilization is an ancient process. Protein HAP2, recognized as the primordial eukaryotic gamete fusogen, is a structural homolog of viral class II fusion proteins. The structural studies using recombinant forms of the ectodomains of Chlamydomonas and Arabidopsis HAP219,20,28 show the same trimer of hairpins conformation observed in the postfusion forms of the viral proteins These studies, along with in vivo evidence indicating a role for the HAP2 fusion loop in fusion[19,21,29], have led to the working model that HAP2 transitions from a prefusion form on minus gametes into homotrimers during the fusion reaction. The gamete membrane fusion reaction is driven by a fusogenic conformational change of HAP2 into trimers, the membrane interaction capacity of the fusion loop is required to couple trimer formation to bilayer merger, and species-specific membrane attachment catalyzes the transition
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