Alkaline Phosphatase Which Lacks Its Own Signal Sequence Becomes Enzymatically Active when Fused to N-terminal Sequences of Escherichia Coli Haemolysin (HlyA)

Fusion of the alkaline phosphatase gene (phoA) which lacks its own signal peptide sequence to the N-terminal region of hlyA, the structural gene for Escherichia coli haemolysin, leads to active alkaline phosphatase (AP). AP activity depends on the length of the N-terminal region of hlyA. An optimum is reached when 100-200 amino acids of HlyA are fused to PhoA but fusion of as little as 13 amino acids of HlyA to PhoA is sufficient to yield appreciable AP activity. When cells are treated with lysozyme most of the AP activity is found associated with the membrane fraction but a substantial amount is also found in the soluble fraction, most of which may represent a periplasmic pool of AP. The soluble portion of AP activity is significantly increased when the cells are disrupted by ultrasonication, which indicates that the fusion proteins are only loosely associated with the membrane and that large parts are already located on the outside of the cytoplasmic membrane. The expected fusion proteins were identified in the soluble and the membrane fractions and their amounts in these fractions correlated well with AP activity.