Karen G. Fleming, Professor, Dept of Biophysics, Johns Hopkins University, will give a talk titled, "From Chaperones to the Membrane with a BAM!"

ABSTRACT

Bacteria have an amazing ability to directionally sort outer membrane proteins (OMPs) in the absence of an external energy source such as ATP. We gained insight into this process through a holistic computational model. Our OMP Biogenesis Model – termed OmpBioM – integrates parameters from experiments both in vivo and in vitro. It incorporates all major periplasmic chaperones at their cellular concentrations, interaction rate constants and considers biological oligomeric states to predict the periplasmic lifetimes, copy numbers and sorting trajectories for OMPs. Using deterministic and stochastic methods we simulated OMP biogenesis under varying conditions replicating biochemical and genetic findings. OmpBioM stochastic simulations reveal that there are hundreds of binding and unbinding events between periplasmic chaperones and unfolded OMPs within an OMP lifetime. These interactions are thermodynamically favored yet kinetically fast suggesting that the periplasmic conditions are near equilibrium with OMPs being “tossed” from chaperone to chaperone. Following this equilibration in the periplasm, the ultimate, rate-limiting step for OMP incorporation into bacterial outer membranes is folding catalyzed by the essential and conserved BAM complex. OmpBioM provides unique insight into this process by predicting the ranges of rates that are possible for this BAM-stimulated reaction. Overall, a finely tuned balance between thermodynamic and kinetic potentials maximizes OMP folding flux through the periplasm, directs them to the native membrane, and minimizes unnecessary degradation. A kinetic “push” prevents OMPs from incorporation into the wrong membrane; OMP sorting is random in the aqueous periplasm; and – once folded with a BAM! – OMPs are thermodynamically favored and kinetically trapped in their native conformations.