Purification and cDNA Cloning of Bovine Liver 5'-Nucleotidase, a GPI-Anchored Protein, and Its Expression in COS Cells

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A glycosylphosphatidylinositol (GPI)-anchored protein, 5'-nucleotidase [EC 3. 1. 3. 5], was released from the membrane of bovine liver by use of phosphatidylinositol-specific phospholipase C (PI-PLC) of Bacillus thuringiensis and purified by several column chromatographies to a homogeneous state. The purified protein has an apparent molecular mass of 61 kDa, as estimated by SDS-polyacrylamide gel electrophoresis. From the partial amino acid sequence of a tryptic peptide, mixed oligonucleotides were synthesized and used to screen a λ gtll liver cDNA library, and one positive clone, pE1, was isolated. Since the insert of the clone lacked the NH₂-terminal coding region, another λgt11 liver cDNA library was screened by using a synthetic probe corresponding to the 5' region of the insert of pE1. Three additional cDNA clones were obtained. Sequencing of these cDNAs revealed an open reading frame that encodes a 574-residue polypeptide with a calculated mass of 63, 084 Da. The predicted structure showed two highly hydrophobic stretches at both ends of the protein, like those of rat and human 5'-nucleotidases. The NH₂-terminal 26 residues comprise a signal peptide and the COOH-terminal hydrophobic stretch may serve as a signal for the posttranslational GPI modification. An expression vector of the cDNA, pSVNT, was constructed in a mammalian expression vector pSVL and the 5'-nucleotidase activity was transiently expressed in COS-1 cells. The expressed activity was about 8 times higher than the pSVL-transfected control activity. PI-PLC released 45% of the transiently expressed 5'-nucleotidase activity, indicating that the cDNA isolated here encodes this enzyme expressed as a GPI-anchored protein.