1. Metabolic regulation of bacterial growth, division, and morphogenesis
Cell growth and division are fundamental processes for cells to survive and proliferate. Therefore, cells developed various strategies to precisely regulate the processes. The single-cell organism E. coli evolved a mechanism involving the Min proteins to mediate placement of the division septum at the midcell. The MinD and MinE proteins oscillate back and forth between two cell poles to partition the division inhibitor MinC to the poles in order to block aberrant polar division. At the molecular level, sophisticated interactions between the Min proteins, and between the Min proteins and the membrane are critical factors to drive the oscillation cycle.
Since the Min protein-mediated division site placement is not essential for cell survival, a puzzling question has been whether there is unanticipated physiological importance that allows the system to be preserved through evolution. Because the membrane interaction is critical to sustain the Min function, we took a systems approach to tackle the question by comparing the inner membrane proteomes of the ∆min mutant and the wild-type strain. Using the quantitative proteomic method, we identified proteins of interest (POIs) whose abundance in the inner membrane was affected in the absence of the Min system. Interestingly, functional analysis of POIs pointed at a link between metabolism and the Min system. In accordance with this observation, we detected changes in the metabolite profile of the mutant. Therefore, we now focus on investigating the mechanisms that underlie the metabolic regulation of cell growth and division, and the coordination with cell morphology and size.
2. Partition of membrane components by the Min proteins
Alongside studying the biological function of the Min proteins, my lab developed an interest in using the membrane mimetic systems in combination with the fluorescence microscopy to study the protein-membrane interactions. By these methods that allow us to appreciate the ‘membrane’ side of the story, we discovered the intrinsic properties of MinE to self-assemble on the membrane surface using the supported lipid bilayers, and to sculpt the membrane and to induce tubulation from liposomes. The in vitro phenomena may reflect the fact that the cell membrane at the division site is actively remodeled and highly unstable due to assembly of the septal proteins that are responsible for growth and division.
Under the same theme, we discovered that dynamic motion of the self-organizing Min proteins can transport the lipid-anchored components on the bilayer surface. The observation assimilates into the knowledge that the MinD/ParA Walker-type ATPase family of proteins can partition various cellular components in bacteria. To follow up, we will investigate the physiological significance of the phenomenon.
3. Cell wall synthesis in response to environmental stresses
The bacterial cell envelope maintains cell integrity, determines morphology, and protects the cell from environmental assaults. It can be the primary target for treating bacterial infection, but on the other side of a coin, it can become a boundary to interfere with the antibiotic treatment. Therefore, understanding the physiology and synthesis of the bacterial cell envelope, including the cell wall, has fundamental importance in formulating strategies to combat bacterial infection.
The bacterial cell wall is synthesized through multiple steps of enzymatic reactions to form a mesh-like structure, peptidoglycan (PG). Although chemical composition, structure, and function of PG have been studied for many decades, the understanding about the spatiotemporal coordination of the protein complexes that are responsible for synthesis and remodeling is still limited. The goal of our study is to investigate the dynamical changes of PG synthesis under different physiological conditions.