title: Uterine milk protein, a novel activin-binding protein, is present in ovine allantoic fluid.
abstract: Activins are pluripotent growth factors that have recently been shown to be present in placental and fetal membrane preparations. Our previous studies have identified and purified activin A from ovine amniotic and allantoic fluids. In this study, ligand blots of side fractions from the isolation of activin A from allantoic fluid suggested the presence of activin-binding proteins other than follistatin. Further purification of one of these fractions involved two sequential reverse phase HPLC steps and a Superose 12HR fractionation. SDS-PAGE revealed a single protein band of 55 kDa, which was identified by NH2-terminal sequencing as ovine uterine milk protein (UTMP), a member of the serine protease inhibitor (serpin) superfamily of proteins. Further binding studies, using ligand blot techniques and Superose 12HR fractionation in the presence of [125I]activin, demonstrated UTMP to be an activin-binding protein with a lower affinity for activin than that of follistatin. A study of the specific binding behavior of UTMP to activin, using surface plasmon resonance, revealed an apparent equilibrium dissociation constant (Kd) of 49 +/- 25 nM, compared with the follistatin-activin Kd of 379 +/- 51 pM. Similar to another activin-binding protein, alpha2-macroglobulin, UTMP was unable to neutralize the bioactivity of activin in a bioassay based on the capacity of activin to inhibit the proliferation of an MPC-11 plasmacytoma cell line. The high concentrations of this protein in uterine fluid during pregnancy and its ability to bind activin suggest that UTMP may act as a low affinity, high capacity binding protein to sequester activin in the local uterine environment.
authors.affiliation: Department of Physiology, University of New England, Armidale, New South Wales, Australia.
authors.forename: J R
authors.initials: JR
authors.lastname: McFarlane
authors.forename: L M
authors.initials: LM
authors.lastname: Foulds
authors.forename: A E
authors.initials: AE
authors.lastname: O'Connor
authors.forename: D J
authors.initials: DJ
authors.lastname: Phillips
authors.forename: G
authors.initials: G
authors.lastname: Jenkin
authors.forename: M T
authors.initials: MT
authors.lastname: Hearn
authors.forename: D M
authors.initials: DM
authors.lastname: de Kretser
mesh_terms: D028341:Activins; D000482:Allantois / Q000378:metabolism*; D000818:Animals; D001826:Body Fluids / Q000378:metabolism*; D005260:Female; D038681:Follistatin; D006023:Glycoproteins / Q000378:metabolism*; D007265:Inhibins / Q000378:metabolism; D011247:Pregnancy; D015843:Serpins; D012756:Sheep / Q000378:metabolism*
chemical_list: D038681:Follistatin; D006023:Glycoproteins; D015843:Serpins; C029708:uterine luminal fluid proteins; D028341:Activins; D007265:Inhibins