Figure 1.
Overview of the rotavirus replication cycle.
During entry into the cell, the outermost protein layer of the triple-layered virion is lost. Polymerase complexes in the core of the resultant double-layered particle produce viral mRNAs that are capped but lack poly(A) tails. Viral proteins and RNAs accumulate in protected sites of the cytoplasm called viroplasms where nascent particle assembly takes place. Interaction of newly formed polymerase complexes with the core capsid protein triggers genome replication, which is followed by addition of the intermediate protein layer of the virion. Double-layered particles bud into the endoplasmic reticulum, acquiring their outer capsid. After release through lysis or trafficking, the attachment spike must be cleaved by trypsin-like proteases in the intestinal lumen to activate the virus for subsequent infection.
Figure 2.
Rotavirus interactions with innate signaling pathways.
Viral nucleic acids may be recognized in a host cell by membrane-bound Toll-like receptors (TLR3) or cytoplasmic RIG-I-like receptors (RLRs). When activated by nucleic-acid binding, RLRs recruit and activate the signaling adaptor molecule IPS-1, which recruits a signaling complex that activates latent cytoplasmic transcription factors such as interferon regulatory factor 3 (IRF3) and nuclear factor-κB (NF-κB). TLR3 activation stimulates the recruitment of the adaptor TRIF, which acts as a platform for a variety of signaling molecules that also phosphorylate IRF3 or NF-κB. When signaled, the C-terminus of IRF3 is phosphorylated, causing a conformational change that leads to dimerization and nuclear translocation. NF-κB is held inactive by inhibitor of NF-κB (IκB). Signals generated during viral infection cause phosphorylation of IκB, followed by ubiquitination (orange circles) and proteasomal degradation mediated by the SKP-CUL-F-box-β-TrCP (SCFβ-TrCP) E3 ubiquitin ligase complex. NF-κB subsequently translocates to the nucleus. IRF3 and NF-κB bind to the IFN-β promoter in a cooperative manner with c-Jun/ATF-2 forming an enhanceosome complex initiating the transcription of IFN-β mRNA. Additional transcription factors, including IRF7, are induced by IFN-β and can also bind to the IFN-β promoter to enhance the transcription of IFN-β and IFN-α genes. PI3K activity may be required for mediating TLR3 and RIG-I signaling by an unknown mechanism (dashed line). PKR responds to dsRNA binding by phosphorylating eIF2α, which ultimately inhibits translation initiation. PKR is also thought to promote the secretion of IFN-β by an unknown mechanism (dashed line). Rotavirus can antagonize innate signaling pathways through several avenues (shown in red), the primary one representing the NSP1-induced degradation of IRF3 and IRF7. Some NSP1 proteins are also known to induce the degradation of β-TrCP. Rotavirus NSP3 can also impede antiviral responses by suppressing the translation of host mRNAs generated from IFN-stimulated genes. By sequestering viral RNAs within viroplasms, the virus can prevent their recognition by PKR, RIG-I, MDA-5, and other sensors that upregulate antiviral responses.