Mutational analysis of the COOH-terminal hydrophobic domain of bovine liver 5′-nucleotidase as a signal for glycosylphosphatidylinositol (GPI) anchor attachment

In order to address the minimum domain of the COOH-terminal hydrophobic region responsible for GPI modification of bovine liver 5′-nucleotidase, we constructed a series of the deletion mutants of the COOH-terminus and expressed them in COS cells. Cells transfected by the deletion mutant of 6 amino acids (-IIILYQ) from the hydrophobic domain (-FSLIFLSVLAVIIILYQ) did not show any elevation of cell surface-associated 5′-nucleotidase activity, whereas the 2 (-YQ) or 4 (-ILYQ) amino acid deletion mutant retained the bovine liver-derived activity on the cell surface as a GPI-anchored protein. Loss of half the hydrophobic domain (6 or 8 amino acids) resulted in accumulation of the activity in the cell. On the other hand, deletion of the whole hydrophobic domain (17 amino acids) or the entire cleaved-off domain (25 amino acids) made the product secreted into the medium. In conclusion, the hydrophobicity of 13 amino acids in length was enough for the GPI modification of the bovine liver 5′-nucleotidase.