Search

Fusion and fission drive all vesicular transport. Although topologically opposite, these reactions pass through the same hemi-fusion/fission intermediate, characterized by a 'stalk' in which only the outer membrane monolayers of the two compartments have merged to form a localized non-bilayer connection. Formation of the hemi-fission intermediate requires energy input from proteins catalysing membrane remodelling; however, the relationship between protein conformational rearrangements and hemi-fusion/fission remains obscure. Here we analysed how the GTPase cycle of human dynamin 1, the prototypical membrane fission catalyst, is directly coupled to membrane remodelling. We used intramolecular chemical crosslinking to stabilize dynamin in its GDP·AlF4(-)-bound transition state. In the absence of GTP this conformer produced stable hemi-fission, but failed to progress to complete fission, even in the presence of GTP. Further analysis revealed that the pleckstrin homology domain (PHD) locked in its membrane-inserted state facilitated hemi-fission. A second mode of dynamin activity, fuelled by GTP hydrolysis, couples dynamin disassembly with cooperative diminishing of the PHD wedging, thus destabilizing the hemi-fission intermediate to complete fission. Molecular simulations corroborate the bimodal character of dynamin action and indicate radial and axial forces as dominant, although not independent, drivers of hemi-fission and fission transformations, respectively. Mirrored in the fusion reaction, the force bimodality might constitute a general paradigm for leakage-free membrane remodelling.

There is much interest in developing synthetic analogues of biological membrane channels with high efficiency and exquisite selectivity for transporting ions and molecules. Bottom-up and top-down methods can produce nanopores of a size comparable to that of endogenous protein channels, but replicating their affinity and transport properties remains challenging. In principle, carbon nanotubes (CNTs) should be an ideal membrane channel platform: they exhibit excellent transport properties and their narrow hydrophobic inner pores mimic structural motifs typical of biological channels. Moreover, simulations predict that CNTs with a length comparable to the thickness of a lipid bilayer membrane can self-insert into the membrane. Functionalized CNTs have indeed been found to penetrate lipid membranes and cell walls, and short tubes have been forced into membranes to create sensors, yet membrane transport applications of short CNTs remain underexplored. Here we show that short CNTs spontaneously insert into lipid bilayers and live cell membranes to form channels that exhibit a unitary conductance of 70-100 picosiemens under physiological conditions. Despite their structural simplicity, these 'CNT porins' transport water, protons, small ions and DNA, stochastically switch between metastable conductance substates, and display characteristic macromolecule-induced ionic current blockades. We also show that local channel and membrane charges can control the conductance and ion selectivity of the CNT porins, thereby establishing these nanopores as a promising biomimetic platform for developing cell interfaces, studying transport in biological channels, and creating stochastic sensors.