Positively charged amino acid residues at the N-terminus of the signal peptide (SP) have been proposed to play a significant role in the initial step of protein secretion in bacteria. To test this hypothesis, Lys(-20) of the Escherichia coli alkaline phosphatase SP was replaced by other amino acid residues, and the effect of these substitutions on protein maturation was studied. The introduction of negatively charged and hydrophobic amino acids resulted in a decrease in secretion efficiency and impaired the SP-APL interaction, whereas His and Tyr had no significant effect. A structural analysis of the SP-APL interaction suggests that the positively charged Lys(-20) determines the stability of the complex.