Sperm Competition and Its Effects on the Evolution of Prostatic Acid Phosphatase in Hominids

Abstract

Selective pressures and their effects on protein structure and function can be found by comparing genes or proteins from different organisms. These pressures can take varying forms. Sexual selection, a form of natural selection, occurs when an organism acquires traits that give it an advantage in mating and producing offspring. An example can be seen in hominid seminal proteins. Hominids exhibit different mating systems which has resulted in seminal proteins being affected by varying degrees of sperm competition. This can result in variation both in the sequences of these genes and the function of the gene products. One such gene, ACPP, is used to generate the protein prostatic acid phosphatase (PAP). This protein is known to dephosphorylate a number of targets in seminal plasma, and it functions as a phosphatase in the acidic pH range. It has been suggested computationally by comparing dN/dS ratios that ACPP may be under strong positive selection in chimpanzees (a species with high sperm competition) and purifying selection in humans and gorillas (both species with weaker sperm competition). We hypothesize that varying degrees of sperm competition have resulted in sequence and functional variation at ACPP/PAP among hominids. ACPPcoding sequences for human, chimp, and gorilla homologs were each inserted into a mammalian expression vector with a C‐terminal His6 tag and transfected into HEK‐293T cells. The phosphatase activities of the PAP homologs were compared using a DiFMUP fluorescent substrate assay. Each protein was tested for pH optima, phosphatase activity at a fixed substrate concentration, and kcat/km. The PAP orthologs exhibited similar pH profiles, with a pH optimum between pH 5.0‐6.0. However, it was found that chimp PAP exhibited elevated phosphatase activity when compared to the other hominids, and this was consistent across a varying pH range. These results suggest that positive selection of chimp ACPPhas resulted in a more efficient PAP protein with elevated activity when compared to human and gorilla PAP, perhaps due to strong sperm competition. Gorilla PAP showed the lowest activity, which is interesting considering that many genes that code for male reproductive proteins have become pseudogenes in gorillas, a species with very low sperm competition. Overall, the results suggest that the varying mating systems of the hominid primates have resulted in functional differences in the PAP homologs of humans, chimpanzees, and gorillas. By bridging together functional and computational techniques such as in this research, we are beginning to understand the effects of specific selective pressures on the function of male reproductive proteins.