Helical disposition of proteins and lipopolysaccharide in the outer membrane of Escherichia coli.

In bacteria, several physiological processes once thought to be the products of uniformly dispersed reactions are now known to be highly asymmetric, with some exhibiting interesting geometric localizations. In particular, the cell envelope of Escherichia coli displays a form of subcellular differentiation in which peptidoglycan and outer membrane proteins at the cell poles remain stable for generations while material in the lateral walls is diluted by growth and turnover. To determine if material in the side walls was organized in any way, we labeled outer membrane proteins with succinimidyl ester-linked fluorescent dyes and then grew the stained cells in the absence of dye. Labeled proteins were not evenly dispersed in the envelope but instead appeared as helical ribbons that wrapped around the outside of the cell. By staining the O8 surface antigen of E. coli 2443 with a fluorescent derivative of concanavalin A, we observed a similar helical organization for the lipopolysaccharide (LPS) component of the outer membrane. Fluorescence recovery after photobleaching indicated that some of the outer membrane proteins remained freely diffusible in the side walls and could also diffuse into polar domains. On the other hand, the LPS O antigen was virtually immobile. Thus, the outer membrane of E. coli has a defined in vivo organization in which a subfraction of proteins and LPS are embedded in stable domains at the poles and along one or more helical ribbons that span the length of this gram-negative rod.