Figure 2.

Entry of RV TLPs into host cells. The entire event of ingress of RV TLPs across the host cell membrane into the cytoplasm is an elaborate process which consists of (1) attachment and (2) post-attachment interactions between the virions and the host cells, followed by (3) endocytosis, (4) endosomal trafficking and penetration of the virus into the host cytosol in the form of DLPs. (1) Host cell membrane sialylated (SA-containing) and fucosylated (HBGAs) glycans act as receptors for RV TLPs. NA-sensitive RV strains require terminal SA-containing glycans as entry receptors; NA-resistant RV strains utilize sub-terminal SA-containing glycans and HBGAs. The RV spike protein VP4, especially by its trypsin-digested domain VP8, interacts with the cellular receptors. (2) Cellular integrins (α₂β₁, α₄β₁, α_Xβ₂, α_Vβ₃) and hsc70 act as post-attachment co-receptors for RV TLPs. A DGE motif of RV-VP5 which is a trypsinized fragment of RV-VP4 is required for interaction with α₂β₁ integrin. Additional VP5 and VP7 regions are required for interactions with hsc70 and integrins. Cellular receptors and co-receptors for RV entry are spatially assembled in lipid raft microdomains of cell membrane. (3) Virions are subsequently internalized by clathrin-mediated or clathrin-independent endocytosis. The GTPase dynamin, cellular actin microfilaments, several actin-binding proteins (Actn4, Diaph, drebrin), and actin microfilament regulatory components (RhoA, Cdc42) modulate the process of rotaviral entry. Viral tethering activates RhoA-dependent stress fiber formation and TJ disruption downstream of RhoA/ROCK/MLC signaling. (4) Endocytosed virions converge in EEs and may progress on to the LEs for uncoating and penetration (late penetrating RVs) or may be released before that (early penetrating RVs). Therefore, early penetrating RVs are sensitive to EE markers EEA1 and Rab5 but not to LE markers Rab7 and Rab9 whereas late penetrating RVs lose infectivity in absence of Rab7 and Rab9. ESCRT complex responsible for ILV formation is also essential for RV entry process. Late penetrating RV strains require protein transporters CD-M6PRs which deliver cathepsins from TGN to LEs. Other protein transporters such as CI-M6PRs and sortilin also regulate infectivity of late penetrating RV strains. TLPs are uncoated to form DLPs which are released from endosomal compartment into host cytosol. Endosomal acidification and calcium concentration can act as triggers for viral nucleocapsid exit. PI3K and ERK signaling downstream of interaction between cell surface receptor/co-receptors and late penetrating RV strains can activate the V-ATPase proton pump resulting in endosomal acidification and viral uncoating. Several inhibitors acting on different steps of RV entry are shown. For additional details, please refer the text