Min-protein oscillations



Min-proteins in branched E. coli



Polar localization of cardiolipin clusters



Membrane-mediated mechanosensitive channel interactions



Cell-wall cracking


My research program employs diverse interdisciplinary methods of inquiry to understand the relationships among cell shape detection, determination, and maintenance in bacteria. Cell shape plays a critical role in regulating many physiological functions, yet little is known about how the wide variety of cell shapes are determined and maintained. Inside the cell, many proteins organize and segregate, but how they detect and respond to the cellular morphology to end up at the right place at the right time is also largely mysterious. I utilize a combination of analytical, computational, and experimental approaches to probe physical mechanisms of shape-related self-organization in protein networks, membranes, and the cell wall. Ultimately, the manipulation of cell shape may provide a direct tool for engineering complex cellular behaviors.

Current topics of interest are:

  • the regulation and mechanics of bacterial cell division
    understanding the roles of Min-protein oscillations and the FtsZ-ring in cell-division site selection and cell-shape maintenance
  • membrane organization
    exploring physical mechanisms for polar localization of the phospholipid cardiolipin and clustering via membrane-mediated protein interactions
  • the structure and synthesis dynamics of the cell wall
    studying the robustness of the cell wall to damage and the origin of cell shape

Bacterial cell division

How does a bacterial cell organize its interior? In the past decade, intracellular fluorescence microscopy has fashioned a new appreciation for the diversity of ways in which proteins organize and segregate on bacterial membranes. Though some targeting anchors are known, cellular symmetry breaking ultimately requires molecular components that self-organize.

The remarkable accuracy of cell division in E. coli and related bacteria is partially regulated by the Min-protein system, which prevents division near the cell ends by oscillating spatially from pole to pole. We have developed a model of the Min system, using only known properties of the proteins, which accurately reproduces the observed oscillations in both rod-shaped and round cells. In particular, we have shown that Min-protein oscillations can select the long axis in nearly round cells, a potentially important factor in division-plane selection in round bacteria such as Neisseria gonnorhoeae. These results suggest that oscillations may provide a general mechanism by which proteins can localize in response to features of cell geometry incapable of localizing individual molecules.

Membrane organization

We have proposed a novel equilibrium mechanism, based on the two-dimensional curvature of the membrane, for spontaneous lipid targeting to the poles and division site of rod-shaped bacterial cells. If one of the membrane components has a large intrinsic curvature, the geometrical constraint of the plasma membrane by the more rigid bacterial cell wall counteracts the attractive interaction between like lipids and leads to microphase separation. We find that the resulting clusters of high-curvature lipids are large enough to spontaneously and stably localize to the two cell poles and septal regions, and could have similar utility to lipid rafts as a stage for targeting proteins involved in a wide variety of biological processes.

Recent evidence of localization of the phospholipid cardiolipin to the poles of bacterial cells suggests that protein targeting may depend on the membranes heterogeneous lipid content. More generally, aggregates of lipids, proteins, and lipid-protein complexes may localize in response to features of cell geometry incapable of localizing individual molecules. Aggregation of proteins can result from the elastic properties of the membrane itself. In recent work, we have demonstrated that two transmembrane proteins in close proximity will influence each other's equilibrium conformation via the resulting local deformations of the membrane, and spatially organize into functional groups based on their geometry.

Cell wall structure and dynamics

Bacterial cells come in a wide variety of shapes and sizes, with the peptidoglycan cell wall as the primary stress-bearing structure that dictates cell shape. In recent years, cell shape has been shown to play a critical role in regulating many important biological functions including attachment, dispersal, motility, polar differentiation, predation, and cellular differentiation. Though many molecular details of the composition and assembly of the cell wall components are known, how the peptidoglycan network organizes to give the cell shape during normal growth, and how it reorganizes in response to damage or environmental forces have been relatively unexplored.

We have introduced a quantitative mechanical model of the bacterial cell wall that predicts the response of cell shape to peptidoglycan damage in the rod-shaped Gram-negative bacterium Escherichia coli. To test these predictions, we used time-lapse imaging experiments to show that damage often manifests as a bulge on the sidewall, coupled to large-scale bending of the cylindrical cell wall around the bulge. The direction of bending confirms the hypothesis of a longitudinal orientation of peptides and a circumferential orientation of glycan strands in the peptidogylcan layer. Our simulations based on our physical model also suggest a surprising robustness of cell shape to damage, allowing cells to grow and maintain their shape even under conditions that limit crosslinking. Finally, we have shown that many common bacterial cell shapes can be realized within the model via simple spatial patterning of peptidoglycan defects, suggesting that subtle patterning changes could underlie the great diversity of shapes observed in the bacterial kingdom.