Viral Morphology:
Rotaviruses are roughly 100 nm particles with three capsids, a feature not seen in any other human virus family (Fields 1918).  The outer two capsids, the outer comprised of VP4 and VP7 and the inner of VP6 (Table 53.4), have a triangulation number of T=13l, another unique aspect of reoviral morphology.  In the outer two layers are channels connecting the innermost capsid (the inner core) to the “outer world” through which newly synthesized mRNA is extruded (Fields 1924).  The inner core houses all the enzymatic functions of the virus and has a T=1 morphology created by dimers of VP2, leading some to call it the unique T=2 (Fields 1925).
paras3b.gif (28300 bytes) 

Capsid morphology of rotavirus


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Electron micrograph of rotavirus

Genome Organization:
The rotaviral genome is made up of 11 segments of double-stranded RNA (dsRNA).  Each segment contains one open reading frame (except for segment 11, which contains two [Fields 1927]) sandwiched between 5’ and 3’ conserved sequences.  The 3’ end of the genome encodes translation enhancers and is not poly adenylated (Fields 1926).  In the capsid, the segments are hydrogen bonded to one another end-to-end and the 5’ end of the positive sense strand has a 5’ cap.  The logistics of packaging a segmented genome can be quite overwhelming, with each segment needing to encode the same sequence to be recognized by the polymerase in addition to a unique segment that will distinguish it from the other segments, a signal possibly encoded in the highly conserved noncoding regions.  In addition to shift, these segments are also able to undergo rearrangement in which bases are added or deleted (Fields 1927).  However, the rearrangement usually leaves the open reading frames intact (Fields 1928).

TABLE 53.4 Rotavirus proteins
Genome Segment Protein Producta Nascent Polypeptide (Mol. Wt× 1000)b Mature Protein Modified Location in Virus Particles Number of Molecules per virionc Ts mutant Groupd Functione
1 VP1 125 - Core 12 C RNA-dependent RNA polymerase, ss-RNA binding, complex with VP3
2 VP2 94 - Core 120 F RNA binding, required for replicase activity of VP1
3 VP3 88 - Core 12 B guanylytransferase, methytransferase, ss-RNA binding, complex with VP1
4 VP4 86.7 Cleaved VP5*(529) VP8*(247)f Outer capsid 120 A Hemagglutinin, cell attachment, neutralization antigen, protease enhanced infectivity, virulence, putative fusion region,
5 NSP1 (NS53) 58.6 - Nonstructural   NA Basic, zinc finger, RNA binding, virulence in mice; interacts with and degrades IRF-3; nonessential for some strains
6 VP6 44.8 - Inner capsid 780 G Hydrophobic, trimer, subgroup antigen, protection (intracellular neutralization ?); required for transcription
7 NSP3 (NS34) 34.6 - Nonstructural   NA Acidic dimer, binds 3′ end of viral mRNAs, competes with cellular PABP for interaction with elF-4G1, inhibits host translation
8 NSP2 (NS35) 36.7 - Nonstructural   E Basic, RNA binding, oligomer, NTPase, helicase, forms viroplasms with NSP5
9 VP7 37.4 cleaved signal peptide glycosylation Outer capsid 780 NA RER intergral membrance glycoprotein, calcium-dependent trimer, neutralization antigen
10 NSP4 (NS20) 20.2 NS29–NS28 uncleaved signal peptide high mannose glycosylation and trimming Nonstructural   NA RER transmembrance glycoprotein, intracellular receptor for DLPs, role in morphogenesis, interacts with viroplasms, modulates intracellular calcium and RNA replication, enterotoxin, secreted cleavage product, protection by antibody, virulence
11 NSP5 (NS26) 21.7 28K, O-glycosylated, phosphorylated (different forms) Nonstructural   NA Basic phosphoprotein, RNA binding, protein kinase, forms viroplasms with NSP2, interacts with VP2 and NSP6
  NSP6 12 not known Nonstructural   NA Interacts with NSP5, present in viroplasms and most virus strains
aThe virion polypeptides are designated as proposed by Mason et al. (439) and modified by Liu et al. (418) and Mattion (449). VP3 is the protein product of gene segment 3, and VP4 is the protein product of gene segment 4. Early studies failed to unequivocally identify a protein product from genome segment 3; hence, the protein product of genome segment 4 was called VP3 in early publications. When the protein product of genome segment 3 was confirmed to be a structural protein located in the inner core, the genome 3 product was designated VP3 and the genome 4 product was renamed VP4 (418). In 1994, the nonstructural proteins were renamed NSP1 to NSP5 to facilitate comparisons among these proteins of different virus strains (449), and this nomenclature is now accepted. The parentheses in this table show the names of the nonstructural proteins as designated previously (NS followed by a number indicating their apparent molecular weight in thousands determined by electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate). NSP6 has been characterized since 1994 (261,686).
bMolecular weights are for SA11 proteins and are calculated from the deduced amino acid sequences from the nucleotide sequence and from the longest potential open reading frame.
cCalculated from structural studies of purified virions (572,573,746).
dNA, no mutant assignment. Four groups of temperature sensitive mutants (tsD, tsH, I, J) are not yet mapped to a genome segment. Data compiled from the following sources: (111,259,437,582.)
eSee text for references.
fThree trypsin cleavage sites are found in SA11 VP4 at amino acids 231, 241, and 247. The indicated mature products are those based on use of only the preferred second cleavage site (420,422).

From Fields 1924