Thesis Defense: Jonathan Lefman, Ph.D. candidate
Advisor: Dr. Sriram Subramaniam
Graduate University: New York University and Graduate Partnerships Program (NIH)

“Architecture of bacterial chemotaxis receptors”

Bacterial chemotaxis is a model system for signal transduction and motility.  Escherichia coli, which has the most studied chemotaxis system, responds to chemical stimuli over a wide concentration range and maintains remarkable sensitivity to small concentration changes.  These responses are mediated by transmembrane chemotaxis receptors, which form complexes with the histidine kinase, CheA.  Kinase activity is modulated by ligand binding and enzymatic methylation/demethylation of specific amino acids in the receptor cytoplasmic domain.  There are several levels of receptor organization that are thought to participate in signal transduction, but it is not known how receptor methylation affects this organization.  To investigate receptor organization, complexes of the serine receptor, Tsr, were explored by three-dimensional transmission electron microscopy.  Our laboratory previously showed that Tsr forms distinct assemblies when over-expressed in E. coli.  Here, sections from E. coli cells that also over-express Tsr were imaged by electron tomography.  Molecular interpretation of the tomographic data show extensive membrane invaginations that are likely stabilized by contacts of Tsr cytoplasmic domains.  Subsequently we explored the effects of receptor methylation on Tsr assemblies using genetically engineered variants of the receptor with fixed methylation states.  The methylation variants, imaged by electron tomography, showed no measurable differences in receptor packing; however averages of tomographic sub-volumes revealed distinct conformational changes due to the methylation state.  Because kinase activity can be regulated by the receptor methylation state, we believe that these conformations represent the first glimpse of kinase modulating states of a chemotaxis receptor.

Date: Thursday, April 17, 2008 Time: 2 p.m. Location: NIH, Building 37, Room 6107