Although biochemically well characterized, the organization of bacterial membranes is not well understood. Now, research published in Molecular Microbiology has revealed that the outer membrane (OM) has a dynamic topography.

Pili and flagella are often localized on the cell surface — laterally, peritrichously or at the poles — but this doesn't indicate OM organization because these appendages are anchored in the periplasm and cytoplasm. To date, information about the organization of intrinsic OM proteins is scarce, and previous studies probing OM protein distribution used fixation techniques that extract proteins from the OM and therefore distort results.

LamB is an integral OM protein that imports maltose and functions as the lambda phage receptor. Gibbs et al. analysed the distribution of LamB in Escherichia coli cells and tracked protein movement using time-lapse microscopy of individual cells. First, lambda phage tails that were covalently hooked up to a fluorophore were used to probe LamB distribution. Almost 40% of cells contained LamB spirals, whereas the remaining cells had bipolar distribution patterns or irregular spirals — none of the cells had uniformly distributed LamB. Tracking LamB movement in single cells showed that a subset of LamB redistributes every 20 seconds along the full cell length, so that LamB spirals are dynamic.

Closer examination of the redistribution patterns, using colloidal gold bound to an epitope tag that was inserted into the LamB protein, revealed two LamB populations — mobile and immobile — that serve to move the spirals along the cell, reminiscent of the FtsZ (cell-division protein) spirals that migrate to the poles from the mid-cell in Bacillus subtilis. The authors suggest that immobile LamB might function as a scaffold on which mobile LamB reassembles spirals as it moves. The mechanism that restrains movement of the immobile LamB population isn't clear, but it is possible that peptidoglycan could anchor LamB. Alternatively, lipids organized into domains — only documented for inner membrane phospholipids in bacteria to date — could exclude LamB from regions of the OM.

The next step in this fascinating story will be to evaluate just how LamB and other OM proteins are localized to understand the dynamic organization of this neglected surface.