Figure 7. Modeling of FHV Transmembrane Protein A into a Spherule.

(A) 3-D map of a single spherule where half of the membrane has been removed to show the interior. As in Figure 4, the spherule membrane is white and the contiguous outer mitochondrial membrane is blue.

(B) Schematic of likely protein A organization within a spherule. Based on the average density of globular proteins, FHV protein A (112 kDa) is modeled in as a green sphere of ∼7 nm in diameter. Based on the average of ∼100 protein A molecules per spherule (Table 1), the figure shows the potential packing arrangement of 50 protein A molecules in half a spherule. The spheres are shown lining the interior surface area of the spherule membrane because protein A is a transmembrane protein [38].

(C) Schematic representation of the structure and organization of the FHV RNA replication complex. Protein A (ptn A; green spheres) forms a shell within the mitochondrial membrane spherule within which RNA synthesis occurs (N = N terminus; C = C terminus). Protein A is also shown as possibly extending into the spherule neck, since it may be a determinant of the relatively constant 10-nm diameter neck. As noted in Figure 1 (white arrowheads), a small fraction of protein A may reside on the outer mitochondrial membrane external to spherules. The diagram shows (+)RNA synthesis (red arrow) from (−)RNA templates (black segmented line), which is the predominant form of FHV RNA synthesis throughout all but the earliest phases of FHV infection (Figure 6A–6D).