Prolipoprotein modification and processing in Escherichia coli

Abstract

An E. coli mutant (lpp-14-1), with an alteration of glycine to aspartic acid at the 14th amino acid residue of the prolipoprotein signal sequence, was previously shown to contain unmodified and unprocessed prolipoprotein in its cell envelope. The wild-type and the lpp-14-1 alleles of the lpp gene were cloned onto a phage .lambda. vector. Two pseudorevertant alleles of lpp-14-1 (14R21 and 6a) were isolated, cloned and sequenced. Amino acid sequences deduced from the DNA sequences of the 2 revertant lipoprotein alleles and biochemical characterization of the revertant lipoproteins show that a conversion of the aspartic acid (residue 14) to asparagine completely restores the modification and processing of the 14R21 revertant prolipoprotein, while a change of the threonine-16 to isoleucine-16 partially enhances the modification and processing of the 6a prolipoprotein, which retains the aspartate-14 substitution. Secondary structure analysis of the revertant prolipoprotein signal sequences according to the Chou and Fasman rules revealed that the specific coil region in residues 14 and 15 and the .beta.-sheet structure in residues 16-18 of signal sequence may be important for prolipoprotein modification. Essential roles of a unique secondary structure and hydrophobicity in residues 14-18 of prolipoprotein signal sequence for the proper recognition by the glyceryl transferase are suggested.