Apolipoprotein, an intermediate in the processing of the major lipoprotein of the Escherichia coli outer membrane

Abstract

A new intermediate (apolipoprotein) in the synthesis of the major lipoprotein of the Escherichia coli outer membrane has been identified. The accumulation of this new form of the lipoprotein was observed when excessive production of lipoprotein was induced or when the membrane fraction containing the prolipoprotein accumulated in the presence of globomycin was incubated at 60 degrees C. The new form of the lipoprotein could be chased into the mature lipoprotein. In addition, from sequential analysis of this new protein by Edman degradation, the NH2 terminus was found to be cysteine, containing a free unmodified amino group and a glyceride-modified sulfhydryl group. These results indicate that this protein is an intermediate in the conversion of glyceride-modified prolipoprotein to the mature lipoprotein. It is believed that the lipoprotein signal peptidase directly cleaves the lipoprotein signal peptide at the peptide bond between the glycine residue at position 20 and the cysteine residue at position 21 of the prolipoprotein. The resulting intermediate, designated here as apolipoprotein, is subsequently acylated at its free amino group to yield the final mature lipoprotein.