Focus

Bacterial cell biology: fundamentals of cytoskeletal dynamics, polarity, and mitosis

Research

Life as we know it requires cellular asymmetry. Without asymmetry, our neurons would not process information, our intestines would not absorb nutrients, and pathogens would not be infectious. No cell is a homogeneous bag of enzymes; instead, cells have complex subcellular architectures with components that are localized to specific places at specific times. The specific architecture of each and every cell allows it to properly grow, divide, differentiate, and communicate. The mechanisms by which cells achieve their subcellular organization are thus fundamental to understanding how life works.

To study such a complicated process, we are initially focusing on a simple cell, the bacterium Caulobacter crescentus. Caulobacter has a unique life cycle during which it divides asymmetrically to produce daughters with different morphologies and fates. A rich variety of cell biological, genetic, biochemical, and genomic techniques make Caulobacter an ideal experimental system for our purposes.

The cytoskeleton plays an essential role in virtually every eukaryotic cellular process examined. Our exploration of Caulobacter cell biology has thus begun with the bacterial cytoskeleton. The absence of a cytoskeletal network was once believed to be a defining distinction between prokaryotes and eukaryotes. However, work in the past few years has shown that bacteria actually possess a full complement of cytoskeletal proteins including actin, tubulin, and intermediate filament protein homologs. The challenge remains to determine what these proteins do in the cell and how they do it.

We have initially focused on the actin homolog, MreB. During the Caulobacter cell cycle, MreB undergoes dynamic rearrangement involving a spiral that collapses into a ring, much like a slinky. Disrupting this dynamic structure dramatically perturbs cell morphogenesis, polarity, and chromosome segregation. Genetic analysis demonstrated that MreB instructs global cell polarity, while temporal studies with an MreB-inhibiting drug and MreB-ChIPs unearthed chromosomal loci that act as a centromere, directing segregation by associating with MreB. Together these results suggest that MreB plays a key role in integrating global positional information.

These findings have opened the door to several exciting avenues that we are currently pursuing. (1) Mechanistically, how does MreB direct polarity and mitosis? We are using genetic screens and biochemical assays to identify and functionally characterize MreB-interacting factors. (2) How is MreB dynamically rearranged? We are implementing new ultra-high-resolution protein labeling and imaging techniques to explore MreB’s dynamic structure. (3) Are there other key cellular regulators? We are performing a functional genomic screen to find all of the Caulobacter proteins with interesting subcellular localizations.

Our group will thus integrate cell biological, genetic, biochemical, biophysical, and genomic approaches to tackle fundamental questions in a simple model system. Eventually, I hope to investigate the similarities between prokaryotic and eukaryotic biology to provide insights into core conserved principles, and to exploit the differences for a new generation of antimicrobial targets.

Honors & Awards