The Assembly of a Structural Lipoprotein in the Envelope of Escherichia coli

Abstract

Abstract A major envelope protein of Escherichia coli was found to exist in two different forms in the envelope. One-third of the protein is covalently linked to the peptidoglycan (bound form) as found by Braun and his co-workers. The remaining two-thirds of the protein are in the envelope but are not covalently linked to the peptidoglycan (free form). In contrast to the free form, the bound form can only be solubilized by 1% sodium dodecyl sulfate (SDS) after lysozyme treatment of the envelope. The bound form thus solubilized shows a slightly higher molecular weight than the free form on SDS acrylamide gel electrophoresis because it has an attached peptidoglycan fragment. When E. coli cells were pulse-labeled with [3H]- or [14C]arginine for 4 min, all of the radioactivity of the protein in the envelope was found in the free form. Chasing with non-radioactive arginine showed that a portion of the free form was converted into the bound form. The ratio of the radioactivity in the free form to that in the bound form reached a constant level after chasing for one doubling time. These results indicate that the free form of the protein is first synthesized and then converted into the bound form. It appears that the reaction between the two forms is reversible and that there is a dynamic equilibrium between the two forms of the protein in the envelope. The conversion from the free form to the bound form was not inhibited by chloramphenicol, amino acid starvation (arginine), or carbonyl cyanide m-chlorophenylhydrazone.