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Biochemical and Structural Characterization of Apolipoprotein A-I Binding Protein, a Novel Phosphoprotein with a Potential Role in Sperm Capacitation

Center for Research in Contraceptive and Reproductive Health, Department of Cell Biology (K.N.J., L.C.D., O.C., C.J.F., J.C.H.), and Department of Molecular Physiology and Biophysics (I.A.S., H.Z., W.M.), University of Virginia, Charlottesville, Virginia 22908; Institute for Clinical Chemistry and Laboratory Medicine (G.S.), University Hospital Regensburg, D-93042 Regensburg, Germany; and Department of Veterinary and Animal Sciences (P.E.V.), University of Massachusetts, Amherst, Massachusetts 01003

Address all correspondence and requests for reprints to: John C. Herr, Ph.D., Department of Cell Biology, University of Virginia, P.O. Box 800732, Charlottesville, Virginia 22908. E-mail: jch7k{at}virginia.edu; or Wladek Minor, Ph.D., Department of Molecular Physiology and Biological Physics, P.O. Box 800732, University of Virginia, Charlottesville, Virginia 22908. E-mail: wladek{at}iwonka.med.virginia.edu.

The physiological changes that sperm undergo in the female reproductive tract rendering them fertilization-competent constitute the phenomenon of capacitation. Cholesterol efflux from the sperm surface and protein kinase A (PKA)-dependent phosphorylation play major regulatory roles in capacitation, but the link between these two phenomena is unknown. We report that apolipoprotein A-I binding protein (AI-BP) is phosphorylated downstream to PKA activation, localizes to both sperm head and tail domains, and is released from the sperm into the media during in vitro capacitation. AI-BP interacts with apolipoprotein A-I, the component of high-density lipoprotein involved in cholesterol transport. The crystal structure demonstrates that the subunit of the AI-BP homodimer has a Rossmann-like fold. The protein surface has a large two compartment cavity lined with conserved residues. This cavity is likely to constitute an active site, suggesting that AI-BP functions as an enzyme. The presence of AI-BP in sperm, its phosphorylation by PKA, and its release during capacitation suggest that AI-BP plays an important role in capacitation possibly providing a link between protein phosphorylation and cholesterol efflux.

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