Distribution of newly synthesized lipoprotein over the outer membrane and the peptidoglycan sacculus of an Escherichia coli lac-lpp strain.

The insertion of newly synthesized lipoprotein molecules into the cell wall of Escherichia coli was studied topographically by immunoelectron microscopy. Lipoprotein was briefly induced with isopropyl-beta-D-thiogalactopyranoside in cells carrying lac-lpp on a low-copy-number plasmid in an E. coli lpp host. Specific antibodies bound to the newly inserted lipoprotein molecules, which were exposed at the cell surface after treatment of the cells with Tris-EDTA, were detected with a protein A-gold probe. The average distribution of the gold particles over the cell surface of noninduced cells was determined for cells induced for 5 and 10 min. Analysis of 250 to 350 cells showed that the distribution of newly synthesized lipoprotein over the cell surface was homogeneous in both cases. The binding of lipoprotein to the peptidoglycan layer was studied by the same technique, and visual inspection again revealed a homogeneous distribution of bound lipoprotein over the entire sacculus surface. It is therefore concluded that free lipoprotein is inserted equally over the entire cell wall of E. coli, while binding to peptidoglycan also occurs over the entire cell surface. The rate of lipoprotein synthesis increased with cell length in nondividing cells, whereas it was constant in cells which had initiated constriction. Analysis of cells having different amounts of lipoprotein in their cell wall revealed that the cell shape depended on the total lipoprotein content of the cell. Cells having no or only a small amount of lipoprotein grew as spheres, whereas cells with increasing numbers of lipoprotein molecules gradually changed their shape to short rods.