The net charge of the first 18 residues of the mature sequence affects protein translocation across the cytoplasmic membrane of gram-negative bacteria

This statistical study shows that in proteins of gram-negative bacteria exp orted by the Sec-dependent pathway, the first 14 to 18 residues of the matu re sequences have the highest deviation between the observed and expected n et charge distributions. Moreover, almost all sequences have either neutral or negative net charge in this region. This rule is restricted to gram-neg ative bacteria, since neither eukaryotic nor grampositive bacterial exporte d proteins have this charge bias. Subsequent experiments performed with a s eries of Escherichia coli alkaline phosphatase mutants confirmed that this charge bias is associated with protein translocation across the cytoplasmic membrane. Two consecutive basic residues inhibit translocation effectively when placed within the first 14 residues of the mature protein but not whe n placed in positions 19 and 20. The sensitivity to arginine partially reap peared again 30 residues away from the signal sequence. These data provide new insight into the mechanism of protein export in gram-negative bacteria and lead to practical recommendations for successful secretion of hybrid pr oteins.