Deep learning insights into the architecture of the mammalian egg-sperm fusion synapse

Abstract

Fertilization is a crucial event in sexual reproduction culminating in the fusion of the haploid sperm and egg to form a new diploid organism. In mammals, egg receptor JUNO and sperm IZUMO1 directly interact to mediate adhesion between gamete membranes; however, their role in fusion remains enigmatic. Here, we used AlphaFold to predict the structure of other extracellular proteins that are essential for fertilization to determine if they could form a complex that may mediate fusion. We first identified TMEM81, whose gene is expressed by mouse and human spermatids, as a protein that has structural homologies with both IZUMO1 and another sperm molecule essential for gamete fusion, SPACA6. Using a set of proteins known to be important for fertilization and TMEM81, we then systematically searched for predicted direct binary interactions using an unguided approach and identified a pentameric complex of three sperm (IZUMO1, SPACA6 and TMEM81) and two egg (JUNO and CD9) proteins. We show that this complex is structurally consistent with both the expected topology on opposing gamete membranes and the location of predicted N-linked glycans that were not considered during prediction with AlphaFold-Multimer. Our model suggests that these proteins could organize to form a synapse-like complex between the sperm and egg membrane at the point of fertilization. Finally, the structural modeling approach described here could be more generally useful to gain structural insights into highly transient protein complexes that are difficult to detect experimentally.