Date of Award

Fall 12-15-2017

Document Type

Undergraduate Honors Thesis

Degree Name

Bachelor of Arts in Biophysics

Department

Physics

Advisor

Dr. Rae Robertson-Anderson

Abstract

Active networks of interlinked protein filaments comprising the cytoskeleton largely control cellular mechanics and cell architecture. By forming cytoskeleton networks that combine motile, semiflexible actin with rigid, supportive microtubules, cells maintain structural integrity and shape while being able to flow and move. To elucidate the complex mechanical processes that arise between interacting networks of actin and microtubules within cells, we create a suite of randomly-oriented, well mixed networks of actin and microtubules by co-polymerizing varying ratios of both proteins in situ. We use optical tweezer microrheology in order to characterize the nonlinear mesoscale mechanics of in vitro co-entangled actin-microtubule composites. To perturb each composite far from equilibrium, we use optical tweezers to displace a microsphere a distance greater than the median contour lengths of the filaments at a speed much faster than their intrinsic relaxation timescales. During perturbation, we simultaneously measure the force the filaments exert to resist the strain and the subsequent force relaxation after strain. We find the presence of a large fraction of microtubules (fT > 0.7) is needed to substantially increase the resistive force, which is accompanied by large heterogeneities in force response. Actin minimizes these heterogeneities by reducing the composites mesh size and supporting microtubules against buckling. Composites also undergo a sharp transition from stress-softening to stress-stiffening dynamics when the fraction of microtubules exceeds 0.7. The induced force following strain relaxes via two time-dependent power-law decays. The first decay phase arises from actin bending fluctuation, with scaling exponents that increase proportionally with the fraction of actin. Alternatively, the second phase is independent of composite composition, with a scaling exponent of ~0.4 indicative of actin filaments reptating out of deformed entanglement constraints.

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