title: The human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase.
abstract: We have demonstrated that the human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase (InsP(4) 5-kinase). The cDNA of the human gene contained a putative open reading frame of 1251 bp encoding 416 amino acids with 83.6% identity compared with the rat protein. The substrate specificity of the recombinant human protein demonstrated preference for Ins(1,3,4,6)P(4) with a catalytic efficiency (V(max)/K(m)) 43-fold greater than that of Ins(1,3,4,5)P(4) and 2-fold greater than that of Ins(1,4,5)P(3). The apparent V(max) was 114 nmol of Ins(1,3,4,5,6)P(5) formed/min/mg of protein, and the apparent K(m) was 0.3 microm Ins(1,3,4,6)P(4). The functional homolog in yeast is Ipk2p, and ipk2-null yeast strains do not synthesize Ins(1,3,4,5,6)P(5) or InsP(6). Synthesis of these compounds was restored by transformation with wild-type yeast IPK2 but not with human InsP(4) 5-kinase. Thus the human gene does not complement for the loss of the yeast gene because yeast cells do not contain the substrate Ins(1,3,4,6)P(4), and the reaction of the human protein with Ins(1,3,4,5)P(4) is insufficient to effect rescue or synthesis of InsP(5) and InsP(6). Therefore the major activity of human InsP(4) 5-kinase is phosphorylation at the D-5 position, and the pathways for synthesis of Ins(1,3,4,5,6)P(5) in yeast versus humans are different.
authors.affiliation: Department of Internal Medicine, Division of Hematology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: Shao-Chun
authors.initials: SC
authors.lastname: Chang
authors.forename: Aimee L
authors.initials: AL
authors.lastname: Miller
authors.forename: Yucheng
authors.initials: Y
authors.lastname: Feng
authors.forename: Susan R
authors.initials: SR
authors.lastname: Wente
authors.forename: Philip W
authors.initials: PW
authors.lastname: Majerus
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015152:Blotting, Northern; D002851:Chromatography, High Pressure Liquid; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D005816:Genetic Complementation Test; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010641:Phenotype; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000096:biosynthesis* / Q000737:chemistry* / Q000378:metabolism*; D010957:Plasmids / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013997:Time Factors; D014018:Tissue Distribution
chemical_list: D011994:Recombinant Proteins; D017853:Phosphotransferases (Alcohol Group Acceptor); C434257:inositol polyphosphate multikinase; C061804:myo-inositol-tetrakisphosphate 5-kinase