Comparison between 102k and 20k Poly(ethylene oxide) Depletants in Osmotic Pressure Measurements of Interfilament Forces in Cytoskeletal Systems

The proliferation of successful, cell-free reconstitutions of cytoskeletal networks have prompted measurements of forces between network elements via induced osmotic pressure by the addition of depletants. Indeed, it was through osmotic pressurization that Tau, an intrinsically disordered protein found in neuronal axons, was recently discovered to mediate two distinct microtubule (MT) bundle states, one widely spaced and a second tightly packed, separated by an energy barrier due to polyelectrolyte repulsions between opposing Tau projection domains on neighboring MT surfaces. Here, we compare interfilament force measurements in Tau coated MT bundles using PEO20k (poly(ethylene oxide), M = 20000), a commonly used inert depletant, and recently published measurements with PEO102k. While force measurements with either depletant reveals the transition between the two bundled states, measurements with PEO20k cannot recapitulate the correct critical pressure (P-c) at which widely spaced MT bundles transition to tightly packed MT bundles due to depletant penetration into widely spaced bundles below P-c. Surprisingly, upon transitioning to the tightly packed bundle state data from both depletants are in quantitative agreement indicative of expulsion of the smaller PEO20k depletant, but only at distances comparable or less than the PEO20k radius of gyration, significantly smaller than the effective diameter of PEO20k. While PEO102k (with size larger than the wall-to-wall distance between MTs in bundles) can more accurately capture the force response behavior at low to intermediate pressures (<10(4) Pa), measurements with PEO20k, beyond the overlap regime with PEO102k, extend the achievable osmotic pressure range into the higher-pressure regime (similar to 5 x 10(4) Pa). The data underscores the importance of the use of polymeric depletants of different sizes to elucidate force response behavior of cytoskeletal filamentous networks over a more complete extended pressure range.