title: Biochemical and structural characterization of apolipoprotein A-I binding protein, a novel phosphoprotein with a potential role in sperm capacitation.
abstract: 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.
authors.affiliation: Center for Research in Contraceptive and Reproductive Health, Department of Cell Biology, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: Kula N
authors.initials: KN
authors.lastname: Jha
authors.forename: Igor A
authors.initials: IA
authors.lastname: Shumilin
authors.forename: Laura C
authors.initials: LC
authors.lastname: Digilio
authors.forename: Olga
authors.initials: O
authors.lastname: Chertihin
authors.forename: Heping
authors.initials: H
authors.lastname: Zheng
authors.forename: Gerd
authors.initials: G
authors.lastname: Schmitz
authors.forename: Pablo E
authors.initials: PE
authors.lastname: Visconti
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Flickinger
authors.forename: Wladek
authors.initials: W
authors.lastname: Minor
authors.forename: John C
authors.initials: JC
authors.lastname: Herr
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000906:Antibodies / Q000302:isolation & purification / Q000378:metabolism; D016632:Apolipoprotein A-I / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism / Q000502:physiology*; D002784:Cholesterol / Q000378:metabolism; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D018076:DNA, Complementary / Q000302:isolation & purification; D004926:Escherichia coli; D006168:Guinea Pigs; D008297:Male; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism / Q000502:physiology*; D010766:Phosphorylation; D019998:Racemases and Epimerases; D017386:Sequence Homology, Amino Acid; D013075:Sperm Capacitation / Q000235:genetics*; D013094:Spermatozoa / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D000906:Antibodies; D016632:Apolipoprotein A-I; D002352:Carrier Proteins; D018076:DNA, Complementary; D010750:Phosphoproteins; D002784:Cholesterol; D017868:Cyclic AMP-Dependent Protein Kinases; C529323:Naxe protein, mouse; D019998:Racemases and Epimerases