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. 2021 Jan 7;13(1):75. doi: 10.3390/v13010075

Figure 1.

Figure 1

Genomic organization, functions of HEV ORFs and crystal structure of ORF4 protein. (A) The (+)-sense HEV genome is capped at the 5′ end, polyadenylated at the 3′ end and can be translated directly by host ribosomes [32]. ORF1 consists of various domains such as methyl transferase (MT), the X domain, papain-like cysteine protease (PCP), hypervariable region (HVR), the Y domain, helicase (Hel), RNA-dependent RNA polymerase (RdRp). Nucleotides 1–979 of HEV ORF1 encode the region responsible for capping the genome that gets translated to a protein of 110 kDa that possesses methyl transferase and guanylyltransferase activity. The protein allows for the transfer of the methyl group to guanosine triphosphate, giving m7-GTP, which gets covalently coupled to the 5′ end of the HEV genome [33]. The X domain binds poly-ADP ribose which is thought to play role in viral replication and/or translation. PCP is known for processing the HEV polyprotein into discrete function units. The Y domain possesses sequence conservation of several motifs within this region across all known HEV genotypes. HVR is rich in proline and plays a structural role as a flexible hinge between adjoining ORF1 regions [34]. Hel domain possesses NTPase activity and is capable of hydrolyzing ATP. RdRp synthesizes sense and antisense RNA transcripts using antisense and sense viral RNA templates, respectively [35]. A bicistronic subgenomic mRNA encodes both the ORF2 and ORF3 proteins of hepatitis E virus [36]. ORF2 encodes for the viral capsid protein (ORF2C) and the free form of the antigen (ORF2S), and ORF3 encodes for multi-functional proteins, small phosphoproteins that is involved in viral morphogenesis, host signaling modification and egress [37,38]. ORF1 begins from 28 nucleotides and terminates at nucleotide position 5109. ORF1 translates into a non-structural protein (pORF1) with 1693 amino acids (aa). ORF2 starts at nucleotide 5147 till nucleotide position 7127. ORF2 translates into a structural protein (pORF2) with 660 aa. ORF3 starts at nucleotide 5133 and terminates at nucleotide 5477. ORF3 translates into structural protein (pORF3) with 114 aa. ORF4 starts at nucleotide 2835 and terminates at nucleotide 3308. ORF4 translates into pORF4 with 158 aa [19]. (B) Predicted 3D structure of the ORF4 protein using I-TASSER server [39,40] depicting the start (Methionine), end (Serine) and 50th aa mutation (from leucine to proline) leading to proteasome-resistant ORF4.