Interacting Proteins on Human Spermatozoa: Adaptive Evolution of the Binding of Semenogelin I to EPPIN

Semenogelin I (SEMG1) is found in human semen coagulum and on the surface of spermatozoa bound to EPPIN. The physiological significance of the SEMG1/EPPIN interaction on the surface of spermatozoa is its capacity to modulate sperm progressive motility. The present study investigates the hypothesis that the interacting surface of SEMG1 and EPPIN co-evolved within the Hominoidea time scale, as a result of adaptive pressures applied by their roles in sperm protection and reproductive fitness. Our results indicate that some amino acid residues of SEMG1 and EPPIN possess a remarkable deficiency of variation among hominoid primates. We observe a distinct residue change unique to humans within the EPPIN sequence containing a SEMG1 interacting surface, namely His92. In addition, Bayes Empirical Bayes analysis for positive selection indicates that the SEMG1 Cys239 residue underwent positive selection in humans, probably as a consequence of its role in increasing the binding affinity of these interacting proteins. We confirm the critical role of Cys239 residue for SEMG1 binding to EPPIN and inhibition of sperm motility by showing that recombinant SEMG1 mutants in which Cys239 residue was changed to glycine, aspartic acid, histidine, serine or arginine have reduced capacity to interact to EPPIN and to inhibit human sperm motility in vitro. In conclusion, our results indicate that EPPIN and SEMG1 rapidly co-evolved in primates due to their critical role in the modulation of sperm motility in the semen coagulum, providing unique insights into the molecular co-evolution of sperm surface interacting proteins.

精液凝固蛋白I（Semenogelin I，SEMG1）可于人精液凝块中检出，同时定位于结合有EPPIN（EPPIN）的精子细胞表面。SEMG1与EPPIN在精子细胞表面的相互作用，其生理学意义在于能够调控精子的前向运动能力。本研究旨在验证如下假说：SEMG1与EPPIN的相互作用界面在人猿总科的演化时间尺度内发生了协同演化，这一过程源于二者在精子保护与生殖适合度中所扮演的角色所带来的适应性选择压力。本研究结果显示，在类人猿灵长类中，SEMG1与EPPIN的部分氨基酸残基存在显著的变异匮乏现象。我们观察到，在包含SEMG1结合界面的EPPIN序列中，存在一处仅人类特有的氨基酸残基变异，即第92位组氨酸（His92）。此外，针对正选择的贝叶斯经验贝叶斯（Bayes Empirical Bayes）分析结果显示，SEMG1的第239位半胱氨酸（Cys239）残基在人类中经历了正选择，这可能与其提升两种相互作用蛋白的结合亲和力的功能有关。我们通过实验证实了Cys239残基在SEMG1结合EPPIN以及抑制精子运动中的关键作用：将SEMG1的第239位半胱氨酸残基突变为甘氨酸、天冬氨酸、组氨酸、丝氨酸或精氨酸后，重组SEMG1突变体与EPPIN的结合能力以及体外抑制人类精子运动的能力均显著下降。综上，本研究结果表明，由于EPPIN与SEMG1在精液凝块中调控精子运动这一关键功能，二者在灵长类中发生了快速的协同演化，为精子表面相互作用蛋白的分子协同演化研究提供了独特的视角。