The primary structure of the N-terminal region of mature alkaline phosphatase is critical for secretion and function of the enzyme

The export signal has been assumed to be localized not only in the signal p eptide of a secreted protein precursor, but also in the N-terminal region o f the mature polypeptide chain. Mutant alkaline phosphatases with amino aci d substitutions of two positively charged residues (Lys or Arg) in this reg ion at different distances from the signal peptide have been studied to tes t this assumption. The efficiency of secretion has been shown to decrease i n mutant proteins with amino acid substitutions in the region of 16-18 amin o acid residues; the closer to the signal peptide is the substitution, the greater is the decrease. A change in the primary structure of the N-termina l domain results also in an increase in the Michaelis constant, which is gr eater the farther is the amino acid substitution from the signal peptide, s uggesting a change in the enzyme function as well.