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. 2012 Aug 21;9:166. doi: 10.1186/1743-422X-9-166

Confusions in orbivirus protein classification

Meik Dilcher 1,, Manfred Weidmann 1
PMCID: PMC3494582  PMID: 22909086

Abstract

An extensive comparative analysis of orbivirus genomes revealed four cases of unclear numeration and protein designation, due to confused reference to protein size or segment size by which they are encoded. A concise nomenclature based on type species, sequence homology and functional characteristics independent of segment or protein size is suggested.

Keywords: Reoviridae, Orbivirus, Protein classification

Background

The genus Orbivirus is one of 15 in the family of Reoviridae containing 22 serogroups (species) and at least 160 different serotypes (strains) [1]. Orbiviruses are transmitted by insects (midges, flies, mosquitoes) or by ticks. Their double-stranded RNA (dsRNA) genomes consist of 10 segments coding for seven structural and at least three non-structural proteins. Orbiviruses have no envelope but a double-shelled icosahedral capsid [2] and include pathogenic agents of wild animals (Epizootic hemorrhagic disease virus (EHDV)), domestic animals (Bluetongue virus (BTV) and African horse sickness virus (AHSV)), and of man (Kemerovo virus (KEMV)) [3]. Type species of the genus is the Culicoides midge transmitted BTV. Insect-borne orbiviruses are much better characterized than tick-transmitted orbiviruses for which few sequences have been described: Broadhaven virus (BRDV, partial) [4], Sandy Bay virus (SBaV, partial (formerly Nugget virus)) [5-8], St Croix River virus (SCRV, complete genome) [9], Great Island virus (GIV, complete genome) [7].

Recently we determined the complete genomes of Tribeč virus (TRBV) and KEMV in a pyrosequencing approach [10] complementing available partial information on segments 1, 2 and 6 of these viruses and of Lipovnik virus (LIPV) [7].

During our extensive comparative analysis of orbivirus genomes we noticed four cases of unclear numeration and protein designation (see Tables 1 and 2). Some laboratories classify orbivirus proteins according to the size of the proteins whereas others use the size of genome segments from which they are encoded.

Table 1.

Comparison of the genome segments and encoded proteins of BTV, YUOV, TRBV, KEMV and GIV

BTV (insect-transmitted) YUOV (insect-transmitted) TRBV (tick-transmitted) KEMV (tick-transmitted) GIV (tick-transmitted)
Segment 1
VP1 (Pol)
150 kDa
RNA-dep.-RNA- Polymerase
Segment 1
VP1 (Pol)
151 kDa
RNA-dep.-RNA- Polymerase
Segment 1
VP1 (Pol)
146 kDa
RNA-dep.-RNA- Polymerase
Segment 1
VP1 (Pol)
146 kDa
RNA-dep.-RNA- Polymerase
Segment 1
VP1 (Pol)
147 kDa
RNA-dep.-RNA- Polymerase
3944 bp
ACR58458
1302 AA
 
3393 bp
YP_443925
1315 AA
 
3892 bp
HQ266581
1284 AA
 
3896 bp
HQ266591
1285 AA
 
3897 bp
ADM88592
1285 AA
 
 
Segment 2
VP2
111 kDa
Outer shell
Segment 2
VP2 (T2)
107 kDa
Inner shell
Segment 2
VP3 (T2)
102 kDa
Inner shell
Segment 2
VP3 (T2)
103 kDa
Inner shell
Segment 2
VP2 (T2)
103 kDa
Inner shell
2953 bp
ACR58459
956 AA
 
2900 bp
YP_443926
940 AA
 
2793 bp
HQ266582
908 AA
 
2792 bp
HQ266592
908 AA
 
2794 bp
ADM88593
908 AA
 
 
Segment 3
VP3 (T2)
103 kDa
Inner shell
Segment 3
VP3
100 kDa
Outer shell
Segment 3
VP4 (CaP)
72 kDa
Capping Enzyme
Segment 3
VP4 (CaP)
72 kDa
Capping Enzyme
Segment 3
VP3 (CaP)
73 kDa
Capping Enzyme
2772 bp
ACR58460
901 AA
2688 bp
YP_443927
873 AA
1935 bp
HQ266583
628 AA
1934 bp
HQ266593
632 AA
1936 bp
ADM88594
635 AA
 
Segment 4
VP4 (CaP)
75 kDa
Capping Enzyme
Segment 4
VP4 (CaP)
74 kDa
Capping Enzyme
Segment 4
NS1 (TuP)
62 kDa
Formes Tubules
Segment 4
VP2
63 kDa
Outer shell
Segment 4
NS1 (TuP)
60 kDa
Formes Tubules
1980 bp
ACR58461
644 AA
1993 bp
YP_443928
645 AA
1734 bp
HQ266584
529 AA
1730 bp
HQ266594
554 AA
1731 bp
ADM88595
531 AA
 
Segment 5
NS1 (TuP)
64 kDa
Forms Tubules
Segment 5
NS1 (TuP)
67 kDa
Forms Tubules
Segment 5
VP2
62 kDa
Outer shell
Segment 5
NS1 (TuP)
60 kDa
Formes Tubules
Segment 5
VP4
62 kDa
Outer shell
1769 bp
ACR58463
552 AA
1957 bp
YP_443929
574 AA
1730 bp
HQ266585
554 AA
1719 bp
HQ266595
529 AA
1722 bp
ADM88596
551 AA
 
Segment 6
VP5
59 kDa
Outer shell
Segment 6
VP5
59 kDa
Outer shell
Segment 6
VP5
59 kDa
Outer shell
Segment 6
VP5
59 kDa
Outer shell
Segment 6
VP5
60 kDa
Outer shell
1638 bp
ACR58462
526 AA
 
1683 bp
YP_443930
535 AA
 
1668 bp
HQ266586
537 AA
 
1668 bp
HQ266596
537 AA
 
1666 bp
ADM88597
537 AA
 
 
Segment 7
VP7 (T13)
39 kDa
Inner shell
Segment 7
NS2 (ViP)
48 kDa
Viral inclusion body matrix protein
Segment 7
NS2 (ViP)
41 kDa
Viral inclusion body matrix protein
Segment 7
NS2 (ViP)
41 kDa
Viral inclusion body matrix protein
Segment 7
VP7 (T13)
40 kDa
Inner shell
1156 bp
ACR58464
349 AA
1504 bp
YP_443931
435 AA
1196 bp
HQ266587
368 AA
1197 bp
HQ266597
368 AA
1181 bp
ADM88598
357 AA
 
Segment 8
NS2 (ViP)
41 kDa
Viral inclusion body matrix protein
Segment 8
VP7 (T13)
40 kDa
Inner shell
Segment 8
VP7 (T13)
40 kDa
Inner shell
Segment 8
VP7 (T13)
40 kDa
Inner shell
Segment 8
NS2 (ViP)
39 kDa
Viral inclusion body matrix protein
1125 bp
ACR58465
354 AA
1191 bp
YP_443932
355 AA
1184 bp
HQ266588
357 AA
1183 bp
HQ266598
357 AA
1172 bp
ADM88599
359 AA
 
Segment 9
VP6 (Hel)
36 kDa
ssRNA and dsRNA binding helicase
Segment 9
VP6 (Hel)
37 kDa
ssRNA and dsRNA binding helicase
Segment 9
VP6 (Hel)
33 kDa
ssRNA and dsRNA binding helicase
Segment 9
VP6 (Hel)
34 kDa
ssRNA and dsRNA binding helicase
Segment 9
VP6 (Hel)
34 kDa
ssRNA and dsRNA binding helicase
1049 bp
ACR58466
329 AA
1082 bp
YP_443933
338 AA
1034 bp
HQ266589
312 AA
1049 bp
HQ266599
317 AA
1056 bp
AMD88600
321 AA
 
Segment 10
NS3
26 kDa
Glycoprotein
Segment 10
NS3
28 kDa
Glycoprotein
Segment 10
NS3
23 kDa
Glycoprotein
Segment 10
NS3
23 kDA
Glycoprotein
Segment 10
NS3
19 kDa
Glycoprotein
822 bp ACR58467 229 AA   825 bp YP_443934 253 AA   705 bp HQ266590 214 AA   707 bp HQ266600 214AA   703 bp ADM88602 171 AA  

Outer and inner shell proteins are labeled in bold. GenBank and SwissProt accession numbers are indicated.

Table 2.

Comparison of the genome segments and encoded proteins of SCRV, PHSV, BRDV and LIPV

SCRV (tick-transmitted) PHSV (isolates only known from horses) BRDV (tick-transmitted) LIPV (tick-transmitted)
Segment 1
VP1 (Pol)
151 kDa
RNA-dep.-RNA-Polymerase
Segment 1
VP1 (Pol)
151 kDa
RNA-dep.-RNA-Polymerase
Segment 1
 
 
 
Segment 1
VP1 (Pol)
146 kDa
RNA-dep.-RNA-Polymerase
4089 bp
YP_052942
1345 AA
3987 bp
YP_460038
1311 AA
3892 bp
ADM88603
1284 AA
 
Segment 2
VP2 (T2)
98 kDa
Inner shell
Segment 2
VP3 (T2)
105 kDa
Inner shell
Segment 2
VP2 (T2)
103 kDa
Inner shell
Segment 2
VP2 (T2)
103 kDa
Inner shell
2747 bp
YP_052943
890 AA
 
2856 bp
YP_460039
925 AA
 
 
P35934
908 AA
 
2793 bp
ADM88604
908 AA
 
 
Segment 3
VP3
74 kDa
Outer shell
Segment 3
VP2
104 kDa
Outer shell
Segment 3
 
 
 
Segment 3
 
 
 
2024 bp
YP_052944
654 AA
 
2747 bp
YP_460040
881 AA
 
 
 
 
 
 
 
 
 
 
Segment 4
VP4 (CaP)
74 kDa
Capping Enzyme
Segment 4
VP4 (CaP)
74 kDa
Capping Enzyme
Segment 4
VP4
63 kDa †
Outer shell
Segment 4
 
 
 
2017 bp
YP_052945
643 AA
1996 bp
YP_460041
646 AA
 
Segment 5
VP5
57 kDa
Outer shell
Segment 5
NS1 (TuP)
64 kDa
Forms Tubules
Segment 5
VP5
53 kDa
Outer shell
Segment 5
 
 
 
1664 bp
YP_052946
517 AA
 
1784 bp
YP_460045
554 AA
 
1658 bp
P21230
480 AA
 
 
 
 
 
 
Segment 6
NS1 (TuP)
58 kDa
Forms Tubules
Segment 6
VP5
59 kDa
Outer shell
Segment 6
NS1 (TuP)
60 kDa
Formes Tubules
Segment 6
VP5
502 AA ††
Outer shell
1657 bp
YP_052947
517 AA
 
1695 bp
YP_460042
529 AA
 
1714 bp
2115436A
537 AA
 
1509 bp ††
ADM88605
 
 
 
Segment 7
NS2 (ViP)
51 kDa
Viral inclusion body matrix protein
Segment 7
NS2 (ViP)
48 kDa
Viral inclusion body matrix protein
Segment 7
VP7 (T13)
40 kDa
Inner shell
Segment 7
 
 
 
1463 bp
YP_052948
462 AA
1613 bp
YP_460046
435 AA
 
P35935
356 AA
 
Segment 8
VP7 (T13)
41 kDa
Inner shell
Segment 8
VP7 (T13)
40 kDa
Inner shell
Segment 8
 
 
 
Segment 8
 
 
 
1256 bp
YP_052949
379 AA
 
1180 bp
YP_460044
353 AA
 
 
 
 
 
 
 
 
 
 
Segment 9
VP6 (Hel)
26 kDa
ssRNA and dsRNA binding helicase
Segment 9
VP6 (Hel)
37 kDa
ssRNA and dsRNA binding helicase
Segment 9
 
 
 
Segment 9
 
 
 
764 bp
YP_052950
232 AA
  1071 bp
YP_460043
334 AA
 
 
 
 
 
 
 
 
 
 
Segment 10
NS3
24 kDa
Glycoprotein
Segment 10
NS3
28 kDa
Glycoprotein
Segment 10
NS3
22 kDa
Glycoprotein
Segment 10
 
 
 
764 bp YP_052951 224 AA   819 bp YP_460047 255 AA     P32555 205 AA          

Outer and inner shell proteins are labeled in bold. GenBank and SwissProt accession numbers are indicated.

†: [12,13], GenBank entry missing.

††: partial sequence.

(i) The inner shell protein T2 for example can be encoded by segment 2 (tick- and mosquito-borne orbiviruses) or segment 3 (Culicoides-borne orbiviruses). This leads to some laboratories labeling this protein VP2(T2) (e.g. GIV), while others designate it VP3(T2) as in the type species BTV. For Peruvian horse sickness virus (PHSV) however, the segment 2 encoded protein is designated VP3(T2) although it is larger (925 amino acids) than the segment 3 encoded VP2 protein (881 amino acids) [11]. To avoid confusion with the outer shell protein VP2 we suggest to exclusively use VP3(T2) for all T2 proteins.

(ii) VP2 and VP2 homologous proteins can be encoded by segments 2, 3, 4 and 5 and are designated VP2, VP3 (YUOV, SCRV) or VP4 (BRDV segment 4 ( [12,13], sequence entry to GenBank missing)), GIV segment 5). Because of the location on the outer capsid and the described sequence similarity with other VP2 proteins, we suggest that the VP4 proteins (BRDV, GIV) as well as the VP3 proteins (YUOV, SCRV) should be uniformly termed VP2, even though tick-borne VP2 proteins have only half the size of insect-borne VP2 proteins [13].

(iii) The capping enzyme VP4(CaP) can be encoded by segment 4 (BTV, YUOV, SCRV etc.) or segment 3 (TRBV, KEMV). In GIV this protein is designated VP3(CaP) [7] and should be renamed VP4(CaP) to avoid confusions with VP3(T2).

(iv) In most cases VP5 is encoded by segment 6 and comprises a component of the outer shell that might be involved in membrane fusion and penetration [14]. TRBV and KEMV also encode VP5 on segment 6. The highest similarity of TRBV VP5 is to LIPV VP5 (95.6%), again encoded by segment 6 [7]. However, VP5 of BRDV is described as encoded by segment 5 [15]. Since in the classification of the viral genome segments bigger segments have smaller segment numbers, and the size of BRDV segment 6 (1714 bp) encoding the NS1(TuP) [16] is larger than the size of BRDV segment 5 (1658 bp) encoding VP5, a reassignment of BRDV segment 5 and 6 (a vice versa switch) seems necessary.

To summarize, it would be much more helpful if the nomenclature of the viral proteins in orbiviruses would reflect the sequence homology and functional relationship rather than protein size or encoding segment size, since the sizes of the orbivirus genome segments sometimes only differ slightly, which leads to even closely related viruses such as TRBV and KEMV encoding VP2 and NS1(TuP) on different genome segments. We therefore suggest the following concise nomenclature based on the type species BTV and on sequence homology and functional characteristics independent of segment or protein size: VP1(Pol), VP2, VP3(T2), VP4(CaP), VP5, VP6(Hel), VP7(T13), NS1(TuP), NS2(ViP), NS3.

Competing interests

The authors declare that they have no competing interests.

Author’s contributions

MD and MW wrote the paper. Both authors read and approved the final manuscript.

Contributor Information

Meik Dilcher, Email: meik.dilcher@medizin.uni-goettingen.de.

Manfred Weidmann, Email: mweidma@gwdg.de.

Acknowledgments

This work was supported by the Federal Ministry of Education and Research (BMBF), grant number 01KI0710, “Research on Zoonotic Infectious Diseases” program, “Emerging arthropode-borne viral infections in Germany: Pathogenesis, diagnostics and surveillance” and the BMBF funded research program “Potential release-oriented biothreat emergency diagnostics (P.R.O.B.E)” for civil security of the German Federal Government as part of the high-tech strategy for Germany.

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