Figure 2.

Comparison of the structural arrangement of the Syt3 Ca²⁺-binding loops with Semliki Forest virus E1 fusion loops and Rhesus Rotavirus VP5 membrane-interacting loops. (a) We superimposed two of the three E1 loops of the Semliki Forest virus E1 protein (blue, PDB ID 1RER²¹) on the Ca²⁺-binding loops of Syt3 (magenta, Ca²⁺ ions are in gray). (b–d) A top view looking from the target membrane onto the proteins is shown in the left panels, and a side view in the right panels. Gray rectangles represent the target membrane. Only the fusion membrane-interacting loops are shown in all panels for clarity. (b) Top view of panel (a) showing only the superimposed loops. (c) We superimposed two of the three membrane-interacting loop regions of the Rhesus Rotavirus VP5 protein trimer (green, PDB ID 1SLQ²²) on the Ca²⁺-binding loops of Syt3 (magenta, Ca²⁺ ions are in gray). (d) Superposition of the Semliki Forest virus E1 protein (blue) and the membrane-interacting loops of Rhesus Rotavirus VP5 protein (green) using all three loop regions. Both E1 of Semliki Forest virus and VP5 of Rhesus Rotavirus are trimers in their post-fusion state. In addition to structural similarities of the Semliki Forest virus fusion loops and the membrane interacting loops of VP5 Rhesus Rotavirus, these loops also share sequence homology²². The fusion/cell entry mechanism of non-enveloped rotavirus is not fully understood, however it is able to mediate cell-cell fusion in vitro⁴⁷.