Lipovitellin 2 beta is the 31 kD Ni(2+)-binding protein (pNiXb) in Xenopus oocytes and embryos

An Ni(2+)-binding protein (pNiXb, 31 kD) present in mature Xenopus laevis oocytes and in embryos from fertilization in N/F stage 42, was isolated and characterized. After oocytes or embryos were fractionated by PAGE, electroblotted onto nitrocellulose, and probed with 63Ni2+, pNiXb was detected by autoradiography. pNiXb, a yolk protein located in the embryonic gut, was purified from yolk platelets by ammonium sulfate precipitation, delipidation, gel filtration chromatography, and HPLC analysis. During these steps, pNiXb copurified with lipovitellin 2. The N-terminal sequence of purified pNiXb exactly matched that of Xenopus lipovitellin 2 beta, deduced from the DNA sequence of the Xenopus vitellogenin A2 precursor gene. Since pNiXb and lipovitellin 2 beta agree in N-terminal sequence, amino acid composition, and apparent molecular weight, they appear to be identical. Based on a metal-blot competition assay, the abilities of metal ions to compete with 63Ni2+ for binding to pNiXb were ranked: Zn2+ approximately Cu2+ approximately Co2+ > Cd2+ approximately Mn2+ > Sn2+. This study shows that Xenopus lipovitellin 2 beta is a metal-binding protein in vitro, and raises the possibility that it may function similarly in vivo.