Fusion surface structure, function, and dynamics of gamete fusogen HAP2.

Area of Science:

• Molecular Biology
• Cell Biology
• Structural Biology

Background:

• HAP2 is a conserved class II gamete fusogen across eukaryotic kingdoms.
• Understanding HAP2's mechanism is key to deciphering fertilization processes.

Purpose of the Study:

• To elucidate the structural and dynamic mechanisms of HAP2-mediated gamete fusion.
• To investigate the role of specific domains and residues in the fusion process.

Main Methods:

• X-ray crystallography to determine the trimeric structure of Chlamydomonas HAP2.
• Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to assess protein dynamics.
• Structure-based comparisons and mutational analysis insights.

Main Results:

• The crystal structure reveals a trimeric HAP2 fusion state with distinct domain arrangements.
• HDX-MS identified unexpectedly dynamic regions on surfaces near the 3-fold axis.
• Fusion helices at the trimer apex splay outwards, creating a cavity, with Arg185 interaction noted.

Conclusions:

• The dynamics and structural arrangement suggest a model where apical domains tilt inward during fusion.
• This inward movement likely facilitates the merging of fusion helices into a common fusion surface.
• HAP2's mechanism involves coordinated domain movements for effective gamete membrane fusion.