ENTRY AND REPLICATION OF MERS-COV IN THE HOST
CELL
An overview of the entry and replication process of MERS-CoV
in the infected host cell is shown in Figure 3 and is discussed
below. The S glycoprotein located on the surface of the
MERS-CoV virion interacts with functional receptor DPP4 to
facilitate viral entry into the host.28,29 The S protein consists
of a globular S1 domain at the N-terminal region; an S2
domain with two heptad repeats (HR), HR1 and HR2; and a
transmembrane domain.30 The S1 domain determines cell
Figure 3 Schematic of the replication cycle of Middle East respiratory syndrome coronavirus (MERS-CoV). MERS-CoV binds to dipeptidyl
peptidase 4 (DPP4) on the host cell through its receptor-binding domain (RBD) in the S1 subunit of the spike (S) glycoprotein, which
leads to virus–cell fusion and the release of genomic RNA into the cytoplasm. Initially open reading frame 1a (ORF1a) and ORF1b are
translated into polyproteins, polyprotein 1a (pp1a) and pp1ab, respectively, which are cleaved by the virus-encoded proteases papain-like
protease (PLpro) and 3C-like protease (3CLpro) into 16 mature nonstructural proteins (nsps). The proteins involved in replication and
transcription are gathered into replication-transcription complexes (RTCs) that associate with double-membrane vesicles (DMVs) derived
from the endoplasmic reticulum (ER). The genomic RNA contains adenylate uridylate (AU)-rich sequences called transcription regulation
sequences (TRSs). If the TRSs are recognized by RTCs, then RNA of subgenomic length for transcription will be generated, otherwise a
full-length template RNA of genomic length for replication will be synthesized. The newly produced genomic RNAs are encapsidated in the
nucleocapsid (N) proteins in the cytoplasm and then transported to the ER–Golgi intermediate compartment (ERGIC) for further assembly.
The S, membrane (M) and envelope (E) proteins are inserted into the membrane of the rough ER (RER), from where they are transported
to the ERGIC to interact with the RNA-encapsidated N proteins and assemble into viral particles. The budded vesicles containing mature
viral particles are then transported to the cell surface for release after maturation in the Golgi bodies. Double-stranded RNAs (dsRNAs) are
partially generated during viral replication. The 4a competes with Toll-like receptor 3 (TLR3) and retinoic acid-inducible gene I product
(RIG-I)-like helicases (RIG-I and melanoma differentiation-associated protein 5 (MDA5)) to bind to dsRNAs and evades the host immune
response.
Therapeutic targets for MERS-CoV
P Durai et al
4
Experimental & Molecular Medicine
tropism and receptor interaction, whereas membrane-fusing
mediators have been identified within the S2 domain.30,31
MERS-CoV binds to DPP4 through a receptor-binding domain
(RBD) located in the S1 subunit. Subsequently, protease
cleavage of the S protein leads to virus–cell fusion and the
release of viral genomic RNA into the host cytoplasm.32,33
ENTRY AND REPLICATION OF MERS-COV IN THE HOSTCELLAn overview of the entry and replication process of MERS-CoVin the infected host cell is shown in Figure 3 and is discussedbelow. The S glycoprotein located on the surface of theMERS-CoV virion interacts with functional receptor DPP4 tofacilitate viral entry into the host.28,29 The S protein consistsof a globular S1 domain at the N-terminal region; an S2domain with two heptad repeats (HR), HR1 and HR2; and atransmembrane domain.30 The S1 domain determines cellFigure 3 Schematic of the replication cycle of Middle East respiratory syndrome coronavirus (MERS-CoV). MERS-CoV binds to dipeptidylpeptidase 4 (DPP4) on the host cell through its receptor-binding domain (RBD) in the S1 subunit of the spike (S) glycoprotein, whichleads to virus–cell fusion and the release of genomic RNA into the cytoplasm. Initially open reading frame 1a (ORF1a) and ORF1b aretranslated into polyproteins, polyprotein 1a (pp1a) and pp1ab, respectively, which are cleaved by the virus-encoded proteases papain-likeprotease (PLpro) and 3C-like protease (3CLpro) into 16 mature nonstructural proteins (nsps). The proteins involved in replication andtranscription are gathered into replication-transcription complexes (RTCs) that associate with double-membrane vesicles (DMVs) derivedfrom the endoplasmic reticulum (ER). The genomic RNA contains adenylate uridylate (AU)-rich sequences called transcription regulationsequences (TRSs). If the TRSs are recognized by RTCs, then RNA of subgenomic length for transcription will be generated, otherwise a
full-length template RNA of genomic length for replication will be synthesized. The newly produced genomic RNAs are encapsidated in the
nucleocapsid (N) proteins in the cytoplasm and then transported to the ER–Golgi intermediate compartment (ERGIC) for further assembly.
The S, membrane (M) and envelope (E) proteins are inserted into the membrane of the rough ER (RER), from where they are transported
to the ERGIC to interact with the RNA-encapsidated N proteins and assemble into viral particles. The budded vesicles containing mature
viral particles are then transported to the cell surface for release after maturation in the Golgi bodies. Double-stranded RNAs (dsRNAs) are
partially generated during viral replication. The 4a competes with Toll-like receptor 3 (TLR3) and retinoic acid-inducible gene I product
(RIG-I)-like helicases (RIG-I and melanoma differentiation-associated protein 5 (MDA5)) to bind to dsRNAs and evades the host immune
response.
Therapeutic targets for MERS-CoV
P Durai et al
4
Experimental & Molecular Medicine
tropism and receptor interaction, whereas membrane-fusing
mediators have been identified within the S2 domain.30,31
MERS-CoV binds to DPP4 through a receptor-binding domain
(RBD) located in the S1 subunit. Subsequently, protease
cleavage of the S protein leads to virus–cell fusion and the
release of viral genomic RNA into the host cytoplasm.32,33
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