Christopher Jones (Sun Lab)

Micromechanics of cells and their environment

Mechanical interactions between cells influence many biological processes such as tissue development, wound healing, and cancer progression. These interactions are mediated by the extracellular matrix (ECM), a fibrous protein network which surrounds and provides structural support for the cells. Using collagen gel as an in vitro model for the ECM, we study the mechanics of the cell environment by using holographic optical tweezers to apply forces to microparticles embedded in the collagen fiber network. We find that variations in the collagen fiber microstructure and distribution of cell adhesion forces cause the mechanical properties to be inhomogeneous at the cellular scale. We also show that as cells deform the surrounding ECM, the micromechanics of the network change in a way that depends on the local strain and distance from the cells. Finally, in addition to reversible elastic deformation, cells can also permanently remodel the ECM. For example, high-density bundles of aligned collagen fibers can form between nearby cells. We are currently exploring the mechanics of this irreversible remodeling of the ECM.

Lee Aspitarte (Minot Lab)

Photocurrent Quantum Yield in Suspended Carbon Nanotube p–n Junctions

We study photocurrent generation in individual suspended carbon nanotube p–n junctions using spectrally resolved scanning photocurrent microscopy. Spatial maps of the photocurrent allow us to determine the length of the p–n junction intrinsic region, as well as the role of the n-type Schottky barrier. We show that reverse-bias operation eliminates complications caused by the n-type Schottky barrier and increases the length of the intrinsic region. The absorption cross-section of the CNT is calculated using an empirically verified model, and the effect of substrate reflection is determined using FDTD simulations. We find that the room temperature photocurrent quantum yield is approximately 30% when exciting the carbon nanotube at the S44 and S55 excitonic transitions. The quantum yield value is an order of magnitude larger than previous estimates.

Andrew R. Popchock (Qiu lab)

The mitotic kinesin-14 KlpA contains a context-dependent directionality switch

Kinesins are microtubule-based motors that catalyze ATP hydrolysis for a variety of essential intracellular processes. Kinesin-14s (i.e. kinesins with a C-terminal motor domain) are commonly considered to be nonprocessive minus end-directed motors that mainly function for mitotic spindle assembly. Here, we show that KlpA – a mitotic kinesin-14 from the filamentous fungus Aspergillus nidulans – is a context-dependent bidirectional motor. KlpA exhibits minus end-directed motility inside microtubule bundles, but on individual microtubules it unexpectedly moves processively toward the plus end. Furthermore, removing the N-terminal nonmotor microtubule-binding domain of KlpA renders the kinesin diffusive on individual microtubules but does not abolish its minus end-directed motility to glide microtubules. Taken together, our results suggest that the nonmotor microtubule-binding domain of KlpA contains a novel dual functionality that not only enables the motor for plus end-directed processive movement on individual microtubules but also acts a switch for controlling its direction of motion.