Elitza Tocheva

Assistant Professor

ElitzaTocheva

Contact Information

Département de stomatologie
Faculté de médecine dentaire
Université de Montréal
Pavillon Roger-Gaudry, room A-224
C.P. 6128, Succ. Centre-ville
Montréal (Québec)
H3C 3J7

.

P 514 343-7715
elitza.tocheva@umontreal.ca
Web Site

Themes

  • Bacterial ultrastructure
  • Electron cryotomography (ECT)
  • Correlated light and electron microscopy

Bacterial Ultrastructure

For decades bacteria were thought of as “bags” of enzymes lacking a cytoskeleton, in contrast to eukaryotes cells where intracellular compartmentalization and establishment of large-scale order has been known for a long time. The development of cryo-preservation of biological samples sparked a new era for Bacterial Cell Biology. In combination with 3-dimensional data acquisition (ECT) we have been able to preserve and image bacterial ultrastructure and elucidate important new mechanistic insights. New super-resolution fLM methods (such as photoactivation localization microscopy, PALM) are being developed that take advantage of single-molecule activation events and deliver resolution beyond the diffraction limit of light. Today we know that bacterial cytoskeletal proteins polymerize into surprisingly diverse superstructures, such as rods, rings, twisted pairs, tubes, sheets, spirals, moving patches or meshes. The vast majority of bacterial filaments and nanomachines, however, remain unknown. These ultrastructures are the driving force in essential cellular processes including cell division, cell shape determination, DNA segregation, secretion or motility. Today a major task is to understand the diverse processes by which bacteria generate intracellular order and perform tasks.

Our work focuses on bridging the scales between individual proteins, macromolecular assemblies and neighboring cells. By applying new correlative fLM and ECT techniques we aim to generate much needed insight into the structure and function of two major macromolecular assemblies: the bacterial cell envelope and DNA segregation machines.

1. The bacterial cell envelope Historically, bacterial cells have been classified on the basis of their ability to retain Gram stain. Gram-negative cells typically have two membranes surrounding a thin layer of peptidoglycan. Gram-positive cells have one membrane and a thicker layer of peptidoglycan. We gained a surprising insight into the relationship between these seemingly different architectures from our ECT studies of sporulation. By imaging a rare endospore-forming Gram-negative bacterium, we found that the inner membrane of the mother cell is transformed into the outer membrane of the germinating spore. This interconversion, and the ability of thick peptidoglycan to be transformed into thin (and vice versa), suggests an evolutionary source of the Gram-negative outer membrane and reveals that monoderm and diderm cell plans may not be so different after all.

2. DNA segregation mechanisms Bacterial and eukaryotic actins share functional properties that have been conserved through evolution. Both polymerize into dynamic filaments that can assemble and disassemble in response to regulatory proteins and nucleotide-induced conformational changes. While bacterial and eukaryotic actins share limited sequence similarity, they have a conserved tertiary structure. However, bacterial actins have adapted to explore a much wider range of sequence variation and partner interactions and therefore display greater variation in filament architecture and dynamics. They are intimately involved in numerous activities ranging from the coordination of cell wall synthesis to the positioning of subcellular structures, and they are grouped into distinct protein families based on their phylogeny and function. Bacteria have many actin-like proteins (Alps) that are either encoded on the chromosome or on mobile genetic elements. Studying these proteins provides an opportunity to characterize the plasticity of bacterial actin and to identify novel mechanisms resulting in a conserved evolutionary function such as DNA segregation.