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Marek’s Disease Virus | 347

Table 12.2 MDV proteins present in the envelope, tegument, and capsid
Envelope
Glycoproteins Other membrane proteins Tegument Capsid
gL (UL1) UL20 UL11 UL6
gM (UL10) UL43 UL13 UL18 (VP23)
gH (UL22) UL14 UL19 (VP5)
gB (UL27) UL16 UL26 (VP21)
gC (UL44) UL17 UL26.5 (VP24)
gN (UL49.5) UL21 UL35 (VP26)
gK (UL53) UL36 (VP1/2) UL38 (VP19C)
gD (US6) UL37
gl (US7) UL41 (VHS)
gE (US8) UL46 (VP11/12)
UL47 (VP13/14)
UL48 (VP16)
UL49 (VP22)
UL49.5
UL51
US2
US3
Glycoprotein complexes: gH-gL, gE-gI, gM-gN.

entry into the cell (Connolly et al., 2011). The gB, one of the can be divided into a unique long (UL), and a unique short
most extensively studied MDV proteins, was used as recombinant (US) regions, each flanked by inverted repeat regions, referred
vaccine against MDV challenge by expressing it in recombinant to as terminal and internal repeats (TRL, IRL, IRS and TRS).
fowlpox virus to protect chickens from developing tumours The organization of the MDV genome resembles that of HSV-1
(Nazerian et al., 1992). The gB and gH-gL play crucial rules in with a co-linear organization of genes in both the UL and US
primary fusion between the virion envelope and the outer nuclear regions. The size of the MDV genome varies by strain with
membrane. The gB functions cooperatively with gH-gL during smaller genomes being found for HVT (159,160 bp) (Afonso
fusion through a triggered conformational change (Chi et al., et al., 2001), followed by MDV-2 (HPRS24: 164,270 bp; SB1:
2013). The gL/gH heterodimer plays a critical role in penetra- 165,994 bp) (Izumiya et al., 2001; Spatz and Schat, 2011), and
tion and cell-to-cell virus infection (Wu, P. et al., 2001), while MDV-1 (GA: 174,000 bp; Md5: 177,874 bp) (Lee et al., 2000;
the complex gM/gN controls viral entry and egress by regulating Tulman et al., 2000) (Table 12.3). Oncogenic MDV-1 viruses
other glycoproteins trafficking and gN modulates gM’s activity on have more genomic coding capacity than avirulent MDV-2 and
membrane fusion (El Kasmi and Lippé, 2015). The gC confers HVT serotypes. The unique genes present in MDV genome are
the first contact between the virion and the cell surface. Further- primarily found in the repeat long regions (TRL and IRL) and at
more, gC can bind to complement component C3b, blocking the the junctions of the UL and repeat long regions. The G + C base
complement cascade (Friedman et al., 1984). Transcriptional composition of the MDV genome also varies by strain (Md5:
analysis of gD, which is essential for herpesviruses attachment and 44%; HPRS24: 53.6%; SB-1: 54% and HVT: 47.5%) and, as with
entry, showed that it is absent in cultured chicken embryo cells, other herpesviruses, the G+C content in the repeat regions is
indicating tissue specific expression and maturation of infectious higher than that in the unique regions (Lee et al., 2000; Tulman et
particles (Tan et al., 2001). Unlike HSV-1, gE–gI heterodimer of al., 2000; Afonso et al., 2001; Izumiya et al., 2001; Spatz and Schat,
MDV is essential for virus growth in cultured cells (Schumacher 2011). Like in other alphaherpesviruses, a-type sequences have
et al., 2001). been identified in direct orientation and in inverted orientation
Studies on the morphogenesis of MDV have been complicated at the genomic termini and at the IRL/IRS junctions (Volkening
by the cell-associated nature of MDV replication, which results and Spatz, 2013).
in non-synchronized infection and low infectivity titres. More
recently, MDV morphogenesis was examined using chicken
embryonic skin cells, and showed that all types of capsid particles MDV genome replication
were present, with the only exception of extracellular enveloped The MDV genome contains two origins of DNA replication and
mature virions. It is hypothesized that MDV is deficient in three about 103 open reading frames (Tulman et al., 2000). Among
crucial steps of herpesviruses morphogenesis: the release from these genes, seven conserved proteins are directly involved in
the nucleus, the secondary envelopment, and the exocytosis pro- DNA synthesis and essential for origin-specific DNA replication,
cess (Denesvre, 2013). while six of them are not required but still involved in nucleo-
tide metabolism. The former group consist of an origin-binding
Genome structure and organization protein with 3′–5′ DNA helicase activity (UL9), a single-strand
The MDV genome consist of a single linear, double-stranded DNA-binding protein (UL29), a heterotrimeric primosome with
DNA molecule, 159,000–180,000 base pairs (bp) long, which 5′–3’ DNA helicase and primase activities (UL5/UL8/UL52),