Roles of the EF-Hand Domain of PLCZ1 in Establishing Species-Specific Ca 2+ Oscillations in Mouse and Rat Fertilization

Abstract Periodic increases in cytosolic calcium concentration (Ca2+ oscillations) during mammalian fertilization induce all the events collectively known as egg activation. The sperm-specific phospholipase C, PLC zeta 1 (PLCZ1) represents the “sperm factor” vital for initiating the persistent Ca2+ oscillations in mammals. Despite sequence conservation, the Ca2+ oscillation-inducing properties of the enzyme differ vastly among species, and this is particularly salient between mouse and rat PLCZ1, where the activities vary at least one order of magnitude in favor of the former. As previously shown, injecting wild-type (WT) rat Plcz1 mRNA into metaphase II (MII) mouse eggs induced delayed Ca2+ oscillations with low specific activity compared to the homologous mouse Plcz1 mRNA. We, therefore, sought to uncover the factor(s) diversifying these enzymes by swapping functional domains between species, creating chimeric PLCZ1s. When injected into mouse MII eggs, mouse Plcz1 mRNA with the whole- or part of the EF-hand domains swapped with the rat showed a substantial reduction in activity compared to WT. Consistently, the opposite exchange enhanced the rat’s enzyme activity. EF-hand domains 1 and 2 seemed to underlie most differences, and mutations of the divergent amino acids within these domains, substitutions for Glu(m-30; r-29) and Gln(m-58; r-57), changed the activity of both species’ PLCZ1s in opposite directions. Collectively, our findings support the view that differences in the sequences of EF-hand domains, especially in several of its charged residues, underpin the distinct PLCZ1 activities between these species, revealing the gametes and species’ adaptability to optimize the fertilization signal and early development.