PT - JOURNAL ARTICLE AU - Feng, Juan AU - Dong, Xianchi AU - Pinello, Jennifer F. AU - Zhang, Jun AU - Lu, Chafen AU - Iacob, Roxana E. AU - Engen, John R AU - Snell, Wiliam J. AU - Springer, Timothy A. TI - Fusion surface structure, function, and dynamics of gamete fusogen HAP2 AID - 10.1101/324194 DP - 2018 Jan 01 TA - bioRxiv PG - 324194 4099 - http://biorxiv.org/content/early/2018/05/16/324194.short 4100 - http://biorxiv.org/content/early/2018/05/16/324194.full AB - HAP2 is a class II gamete fusogen in many eukaryotic kingdoms. A crystal structure of Chlamydomonas HAP2 shows a trimeric fusion state. Domains D1, D2.1 and D2.2 line the 3-fold axis; D3 and a stem pack against the outer surface. Surprisingly, hydrogen-deuterium exchange shows that surfaces of D1, D2.2 and D3 closest to the 3-fold axis are more dynamic than exposed surfaces. Three fusion helices in the fusion loop of each monomer expose hydrophobic residues at the trimer apex that are splayed from the 3-fold axis, leaving a solvent-filled cavity between the fusion loops in each monomer. At the base of the long fusion loop, Arg185 docks in a carbonyl cage. Comparisons to other structures, dynamics, and the greater effect on Chlamydomonas gamete fusion of mutation of axis-proximal than axis-distal fusion helices suggest that the apical portion of each monomer could tilt toward the 3-fold axis with merger of the fusion helices into a common fusion surface.