Membrane nanotubes or tunneling nanotubes (TNTs) that connect cells have been recognized as a previously unidentified pathway for intercellular transport between distant cells. However, it is unknown how this delicate structure, which extends over tens of micrometers and remains robust for hours, is formed. Here, we found that a TNT develops from a double filopodial bridge (DFB) created by the physical contact of two filopodia through helical deformation of the DFB. The transition of a DFB to a close-ended TNT is most likely triggered by disruption of the adhesion of two filopodia by mechanical energy accumulated in a twisted DFB when one of the DFB ends is firmly attached through intercellular cadherin-cadherin interactions. These studies pinpoint the mechanistic questions about TNTs and elucidate a formation mechanism.

UvrD is a prototypical Superfamily 1 (SF1) helicase from E. coli involved in DNA repair. We report herein the directmeasurement of the stepping behavior of UvrD at thesingle-molecule level as it unwinds and re-zips a DNA hairpin usinghighresolution optical tweezers. We measure an averageunwinding and re-zipping step size of 3 base pairs, but also observe large variation in step size.Analysis of the step size distribution reveals the presence of multiple sub-steps smaller than 3 bp,suggesting that 3 bp is not the elemental unit of step sizefor UvrD. Further analysis of the stepping kinetics points to asingle rate-limiting event per step for unwinding at low ATPconcentrations. Based on our results and previous studies,we propose a mechanism in which UvrD unwinds DNA 1 bpat a time but sequesters the nascent single strands andreleases them only after later rounds of unwinding. This model is supported by molecular dynamics simulationswhich have identified several basic amino acid residueswithin the motor core of the protein that interact with ahigh probability with the ssDNA backbone.