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Organisms have evolved stress-inducible defense responses such as the P-glycoprotein (P-gp)-mediated efflux system to maintain chemical homeostasis in cells for both endogenous and xenobiotic compounds. However, despite the extensive focus on the potential interactions of P-gp with small molecules, the effect of nanoparticles on this transporter is scarcely reported. Thus in this work, in vitro experiments combined with molecular dynamics (MD) simulations were carried out to investigate the interactions of the multidrug resistance (MDR) protein P-gp with fullerene (C 60 ), one of the most important nano-drug carriers. Upon exposure to fluorescence-labeled C 60 (0-70 mg mL À1 ) for 2 h, significant accumulation of C 60 is found in both the K562S and K562R cells, suggesting the incapability of P-gp to induce the efflux of this nanoparticle. In addition, in vitro inhibition assays also reveal that C 60 does not obviously hinder Pgp-mediated rhodamine-123 transport in both K562S and K562R cells. The theoretical simulations further reveal the mechanism involved in C 60 -P-gp interactions, i.e., the binding of C 60 barely induces the conformational changes of P-gp with RMSD of $4.8 A and radius of gyration of $41.5 A, and also no theoretical evidence shows that the C 60 acts as a substrate or inhibitor of P-glycoprotein. These results demonstrate the potential of C 60 as a good carrier candidate for MDR-targeted drug delivery, since organisms probably have not evolved to recognize this nanoparticle.

Connexin (Cx) hemichannels controlling an exchange of ions and metabolites between the cytoplasm and extracellular milieu can be modulated by the variation of intracellular pH during physiological and pathological conditions. To address the mechanism by which the pH exerts its effect on hemichannels, we have performed two 100-ns molecular dynamics simulations of the Cx26 channel in both acidic and neutral states. The results show that: 1) transmembrane domains undergo clockwise motions around the pore axis under both acidic and neutral conditions, while extracellular segments keep stable. 2) Under neutral condition, Cx26 has a tightly closed configuration that occurs through the assembly of N-terminal helix (NTH) region. This shows a constriction formed by the interhelical interactions of Asp2 and Met1 from neighboring NTH, which shapes the narrowest segment (pore radius<2Å) of the pore, preventing the passage of ions from the extracellular side. This indicates that Asp2 may act as a channel gate. 3) Under the acidic condition, the constriction is relieved by the protonation of Asp2 causing interruption of interhelical interactions, Cx26 has a flexibly opening pore (pore radius>4.5Å) around NTH region, allowing the passage of chloride ions unimpeded by the side-chain Asp2. While in the extracellular part two chloride ions interact with the side-chain Lys41 from three subunits. Finally, we provide a plausible mechanism of pH-dependent gating of hemichannel that involves protonation of the aspartic residues, suggesting that the pH sensitivity of hemichannel permeability is a sophisticated mechanism for cell regulating ion permeation.