Mechanism of Membranous Tunnelling Nanotube Formation in Viral Genome Delivery
(A) Sequential steps of PRD1 infection and tube formation. Step 1, PRD1 particles approach the cell surface in a random orientation. Step 2, after binding to the cellular receptor, partial rolling/tumbling of the particle due to solvent movement and/or binding of further viral protein P2 or secondary binders such as P5 proteins to cellular receptors guide the re-orientation of the unique vertex almost orthogonal to the cell. De-capping of the cell-interacting vertex triggers the membrane reshaping as a consequence of the loss of vertex-stabilizing protein P16 interactions and the influx of components of the extracellular milieu through the opened vertex (cf., change in osmolarity). Step 3, the vesicle transformation leads to the tube polymerization and piercing of the cell envelope as well as digestion of the peptidoglycan layer by virion-associated enzymes . Once in the cytoplasm, the tip of the tube unplugs and the DNA is translocated relative to the tube from the vesicle into the cytoplasm. (A, Inset) Matching of the ring-like structure (black arrowheads) within the capsid region of the gate volume (white-smoke). (B, Left) Isosurface representation of the averaged model tube 2 (n = 33, Figure 5C, Left) with the distinctive ring-like structure nearby the top end of tube (low-pass band filtered to 5.0 nm and contoured at 0.8σ; gold) with a model of B-DNA fragment manually docked within the channel viewed perpendicular to the tube long axis. (B, Centre) As left but displayed at higher contouring level (1.4σ) (Top) and 90° rotated (Bottom). (B, Right) As centre but viewed through a cutting-plane halfway into the tube (Top) and with a 90° rotated view of the tube cut-through above the tip-end (Bottom). For a superimposition of the averaged density of tube 2 onto average tube 3, see Figure S6. (C) Putative tube's architecture. (C, Top) Stereo-view of a portion of the averaged tube 2 density (as mesh contoured at 1.4σ; 90° rotated relative to B, Top) cut-through its longitudinal axis with manually fitted a lipid bilayer model (cyan) and a membrane protein (blue). (C, Bottom) Stereo-view of the architecture of the tube as depicted by filling the averaged density of tube 2 with a net of atoms (gold) representing the membrane bilayer in turn decorated by scaffolding membrane proteins (blue). These proteo-lipid upright strands are connected by weaker lateral contacts that confer to the highly curved tube wall the fenestrated appearance.