P4-ATPase flippase which catalyzes the hydrolysis of ATP coupled to the transport of aminophospholipids from the outer to the inner leaflet of various membranes and ensures the maintenance of asymmetric distribution of phospholipids (PubMed:14975727, PubMed:19017724). Phospholipid translocation seems also to be implicated in vesicle formation and in uptake of lipid signaling molecules (PubMed:14975727). May be responsible for the maintenance of asymmetric distribution of phosphatidylserine (PS) in spermatozoa membranes (PubMed:14975727). Involved in acrosome reactions and binding of spermatozoa to zona pellucida (PubMed:19017724). ATP + H2O + phospholipidSide 1 = ADP + phosphate + phospholipidSide 2. a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) + ATP + H2O = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) + ADP + H(+) + phosphate Expressed in testis, specifically in spermatids within seminiferous tubules (at protein level). Developmentally regulated in testis. Expression is first detected at 17 days after birth and is later up-regulated up to adulthood amd maintained thereafter. Disrupted normal fertilization by sperm; sensitivity to sperm concentration and the time required to fertilize an egg is increased; the number of spermatozoa bound to zona pellucida is decreased. Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IV subfamily.