Both a short hydrophobic domain and a carboxyl-terminal hydrophilic region are important for signal function in the Escherichia coli leader peptidase

Leader peptidase, typical of inner membrane proteins of Escherichia coli, does not have an amino-terminal leader sequence. This protein contains an internal signal peptide, residues 51-83, which is essential for assembly and remains as a membrane anchor domain. We have employed site-directed mutagenesis techniques to either delete residues within this domain or substitute a charged amino acid for one of these residues to determine the important properties of the internal signal. The deletion analysis showed that a very small apolar domain, residues 70-76, is essential for assembly, whereas residues that flank it are dispensable for its function. However, point mutations with charged amino acid residues within the polar sequence (residues 77-82) slow or abolish leader peptidase membrane assembly. Thus, a polar region, Arg-Ser-Phe-Ile-Tyr-Glu, is important for the signal peptide function of leader peptidase, unlike other signals identified thus far.