In vitro polymerization of microtubules with a fullerene derivative.

Fullerene derivative C(60)(OH)(20) inhibited microtubule polymerization at low micromolar concentrations. The inhibition was mainly attributed to the formation of hydrogen bonding between the nanoparticle and the tubulin heterodimer, the building block of the microtubule, as evidenced by docking and molecular dynamics simulations. Our circular dichroism spectroscopy measurement indicated changes in the tubulin secondary structures, while our guanosine-5'-triphosphate hydrolysis assay showed hindered release of inorganic phosphate by the nanoparticle. Isothermal titration calorimetry revealed that C(60)(OH)(20) binds to tubulin at a molar ratio of 9:1 and with a binding constant of 1.3 ± 0.16 × 10(6) M(-1), which was substantiated by the binding site and binding energy analysis using docking and molecular dynamics simulations. Our simulations further suggested that occupancy by the nanoparticles at the longitudinal contacts between tubulin dimers within a protofilament or at the lateral contacts of the M-loop and H5 and H12 helices of neighboring tubulins could also influence the polymerization process. This study offered a new molecular-level insight on how nanoparticles may reshape the assembly of cytoskeletal proteins, a topic of essential importance for illuminating cell response to engineered nanoparticles and for the advancement of nanomedicine.

Concerted diffusion of lipids in raft-like membranes.

Currently, there is no comprehensive model for the dynamics of cellular membranes. The understanding of even the basic dynamic processes, such as lateral diffusion of lipids, is still quite limited. Recent studies of one-component membrane systems have shown that instead of single-particle motions, the lateral diffusion is driven by a more complex, concerted mechanism for lipid diffusion (E. Falck et al., J. Am. Chem. Soc., 2008, 130, 44-45), where a lipid and its neighbors move in unison in terms of loosely defined clusters. In this work, we extend the previous study by considering the concerted lipid diffusion phenomena in many-component raft-like membranes. This nature of diffusion phenomena emerge in all the cases we have considered, including both atom-scale simulations of lateral diffusion within rafts and coarse-grained MARTINI simulations of diffusion in membranes characterized by coexistence of raft and non-raft domains. The data allows us to identify characteristic time scales for the concerted lipid motions, which turn out to range from hundreds of nanoseconds to several microseconds. Further, we characterize typical length scales associated with the correlated lipid diffusion patterns and find them to be about 10 nm, or even larger if weak correlations are taken into account. Finally, the concerted nature of lipid motions is also found in dissipative particle dynamics simulations of lipid membranes, clarifying the role of hydrodynamics (local momentum conservation) in membrane diffusion phenomena.