A selection of our research projects.

Our view: Basic unit of the spindle as a superposition of two active materials.

How does the cell solve the control problem of transporting long, entangled critical-information-containing molecules? The key to how the cell solves this problem lies in the properties of its special polymer/motor based materials. We design new in vitro systems that provide key knowledge into the role of forces in the events of chromosome segregation. We use our findings on the in vitro systems to help understand the unique mechanics of these polymer/motor based materials, and use this knowledge to interpret our experiments in vivo where the two active systems couple during cell division.

Active Entangled DNA

Active Cytoskeleton

Sensing of force-dependent transcription fidelity in living cells

Another active process, one that is highly robust, is the "routine" process of gene transcription. Error-free transcription requires the RNA polymerase to maintain proper registry with the track (DNA) message. Errors are a natural part of RNA transcription. One type of error involves slippage of the registration by one base, believed to involve the temporary uncoupling of polymerase translocation from polymerization (transcript elongation). This type of error is more prone to occur if the motor is paused, or idling, at one site on the track. Transcription errors are normally extremely rare and difficult to detect. Under duress, this and other machinery on which the cell absolutely relies - normally robust - should begin to exhibit errors more frequently. Together with Dr. Jeffrey Strathern at NIH, we have developed cells that are genetically engineered to `display' erroroneous transcription, using a tool of genome editing, Cre recombinase, to cause the cells to switch from red- to green-fluorescent protein expression in the event of transcriptional slippage error.

Molecular Motor Kinetics in Living Cells

Understanding the physical principles that underlie the motility and load-bearing capacities of ensembles of kinesin-5 motors involved in cell division and neural growth is important to basic understanding of these processes. To study this problem we carry out three-dimensional time-lapse confocal microscopy in living cells and detailed image analysis, reaching a wealth of data and a resolution approaching that which characterizes single-molecule measurements on motor proteins. In so doing we address an important gap in the field of molecular motors and mitosis. We see, for example, that the separation between the two poles in anaphase can serve as a good reaction coordinate to follow the kinetics of the collective, ensemble dynamics of kinesin-5 motors that drive chromosome separation during mitosis. We use knock-out mutants to select cells expressing single populations of kinesin-5 motors, to create minimal living systems with respect to these motors.