FIGURE 1.

The organization of the potexvirus genome. (A) The RNA-dependent RNA polymerase (RdRp, replicase) gene contains a methyltransferase domain (MET), a helicase domain (HEL), and an RNA polymerase domain (POL). The three genes of the triple gene block (TGB) are partially overlapped. Arrows indicate subgenomic (sg) RNAs for expression of TGBs. (B) The organization of the three TGB genes. TGB1: The first TGB ORF encodes the TGB1 protein and has a helicase-like domain (HELD), which contains seven typical motifs of a general helicase (I, Ia, II, III, IV, V, and VI; dark boxes). TGB2: the TGB2 protein is encoded in the second TGB ORF and has two transmembrane domains (dark boxes). The GDx₆GGxYxDG sequence is conserved in TGB2-encoding viruses. TGB3: The TGB3 protein is encoded by the third TGB ORF and contains a transmembrane domain (dark box). Among the TGB3-encoding potexviruses, the TGB3 gene has a conserved C(x₅)G(x_6-9)C sequence. (C) 3-D structural models of TGB2 and TGB3 protein in the endoplasmic reticulum (ER). Potexvirus TGB2 protein integrates into ER membranes to form a U-like structure (with the central loop exposed to the ER lumen and with both the N- and C-terminus located in the ER cytoplasm) and has two conserved cysteine residues (Cys₁₀₉ and Cys₁₁₂) in the C-terminal region, which has a positive net charge (TGB2). Potexvirus TGB3 has an N-terminus that has a negative net charge and is exposed to the ER lumen and a C-terminus that is exposed to the ER cytoplasm (Morozov and Solovyev, 2003).