A novel aminophospholipid transporter exclusively expressed in spermatozoa is required for membrane lipid asymmetry and normal fertilization.

Through the use of a functionally unbiased signal peptide trap screen, we have discovered an ATP-dependent aminophospholipid transporter that is exclusively expressed in the acrosomal region of spermatozoa; it is about 62% similar to the flippase, FIC1. We disrupted the transporter gene and found that the size of litters from male null mice was slightly smaller than found with wild-type males. Sperm morphology and motility were the same between null and wild-type littermates, but agents (merocyanine and annexin) that measure phospholipid packing or phosphatidylserine (PS) in the outer membrane leaflet showed that PS already existed in the outer leaflet of null spermatozoa before sperm capacitation. Fertilization rates were normal when null spermatozoa were added to zona pellucida-free eggs, but in the presence of the extracellular matrix, fewer transporter(-/-) spermatozoa bound tightly or penetrated the zona pellucida (ZP), and fewer underwent acrosome reactions. In vitro fertilization was compromised, especially at early time points or at low sperm concentrations after mixing null spermatozoa and eggs. Thus, a new aminophospholipid transporter expressed exclusively in spermatozoa is critical for normal phospholipid distribution in the bilayer, and for normal binding, penetration, and signaling by the zona pellucida.