Primate proteomic composition of seminal plasma and prostate-specific transglutaminase activity in relation to sexual selection.

Humans (Homo sapiens), chimpanzees (Pan troglodytes), and gorillas (Gorilla gorilla) have diverse mating systems with varying levels of sperm competition. Several seminal plasma genes have been claimed to evolve under positive selection, while others are altered or lost. This study aims to identify biologically relevant differences among seminal plasma proteomes of primates in relation to mating systems and previous genomic studies. Seminal plasma from three individuals of each species were run in triplicate in shotgun liquid chromatography – tandem mass spectrometry (LC-MS/MS) and confirmed with Western blots. Over 7,000 peptides were identified across all individuals; 168 proteins were identified with high confidence, 70 seminal plasma proteins were identified for human, 64 proteins for chimpanzee, and 34 proteins for gorilla. The gorilla seminal plasma proteome has higher variation among individuals and many proteins involved in semen coagulation and liquefaction have been lost. Chimpanzees have approximately 7-fold higher prostate specific transglutaminase (TGM4) expression than humans. TGM4 was not detected in gorillas, supporting pseudogenization of this gene. The structural semenogelin proteins, SEMG1 and SEMG2, were detected in high abundance in only one of three gorilla individuals, and in all three human and chimpanzee individuals. Chimpanzees have significantly higher expression of SEMG1 (~2.5-fold) compared to human; whereas, they only produce a small amount of SEMG2; ~6.5 –fold less than humans. Chimpanzees have roughly 34-fold higher expression of a serine protease inhibitor, SERPINA3 (Serpin Family A Member 3), than humans. SERPINA3 paralogs, SERPINA1 and SERPINA5, also have increased expression (~2.5 –fold) compared to human, and only SERPINA1 was detected in gorilla. SERPINAs may delay protease dissolution of the copulatory plug in chimpanzees. Recombinant human TGM4 and the reconstructed ancestral TGM4 sequence of our last common ancestor (LCA) with chimpanzees (the human-chimpanzee ancestor) proteins were produced and incubated with casein and monodansylcaverdine to determine enzymatic activity. The human-chimpanzee ancestor TGM4 had higher activity compared to human TGM4. Considering the importance of TGM4 in semen coagulation and copulatory plug formation in chimpanzee, the increased activity of the human-chimpanzee ancestor TGM4 may be indicative of elevated female promiscuity of our LCA, perhaps similar to a chimp-like mating system