Fig. 3.

Model on the biogenesis of the crescent. 1. Viral membrane-formation (orange) is initiated in an area of pre-assembled honey-comb like scaffold protein, composed of the gene product of D13 of VACV, that forms rod-like structures (red) and that is surrounded by smooth closed membrane structures (yellow). These structures rupture to form open membrane sheets. 2. The open sheets are transported to the growing crescent membrane shaped by the assembly of the scaffold on its convex side. The crescent grows as more of the open membranes, transported from their site of rupture, fuse with the crescent membrane. The newly formed crescent associates with the virosome (green), which is composed of soluble viral proteins. We speculate that a protein-lipid complex (black dot) stabilizes the ends of the open membrane sheet and prevents re-sealing (fusion). Disassembly of this complex could trigger fusion of ruptured membranes with the growing crescent membrane or the sealing of the IV after DNA-uptake. 3. The sphere remains open and forms a pore of heterogeneous size through which the viral DNA (blue) is inserted into the particle. 4. After DNA take-up, the membrane sphere is sealed and forms a closed compartment, likely after disassembly of the putative lipid/protein complex at the membrane ends (black dots). IV: immature virion, IVN: immature virion containing the nucleoid (from Chlanda et al., 2009 reproduced with permission).