The most abundant protein components of the outer shell are the MCPs. Capsids comprise 11 pentons and 150 hexons that are divided into five and six MCP copies, respectively. In HCMV, the viral gene UL86 encodes the major capsid protein pUL86, composed of 1370 amino acids (Figure 3). pUL86 is essential for HCMV capsid formation and the generation of viral progeny. This is one of the most conserved proteins in herpesviruses [12] (Figure 3, Table 1). The MCP domains are organized into two regions formed by distinct domains (Figure 3): a tower region composed of the upper (amino acids (aa) 481 to 1031), channel (389 to 480 and 1322 to 1370), buttress (1107 to 1321) and helix-hairpin (190 to 233) domains, which make up the penton and hexon capsomer protrusions, and a floor region composed of the N-lasso (1 to 59), dimerization (291 to 362) and Johnson-fold (60 to 189, 234 to 290, 363 to 397 and 1032 to 1106) domains [13]. Superposition of the cryo-electron microscopy structure of HCMV MPC (pUL86) and HSV-1 MCP (VP5) has shown strong similarities. Differences observed are more pronounced in the N-lasso and upper domain, in particular at the top of the upper domain where SCP binding occur (Figure 3). This similarity of secondary and tertiary structures was also demonstrated through structural alignments based on the VP5ud model and the HCMV MCP model’s upper domain [9]. For HHV-8, although the structures are similar, a loop in the HSV-1 MCP is replaced by a helix in HHV-8, producing a groove into which an SCP binds [13].

The UL48.5 gene encodes the smallest HCMV protein. This protein of 75 amino acids, essential for HCMV viral growth, would participate in the cohesion of the capsid by coating the ends of the hexons and pentons [14]. Unlike in HCMV, where SCPs bind hexons and pentons so that exactly one SCP binds each MCP, the HSV-1 SCP binds only hexon MCPs [9,15]. Moreover, the SCP in HSV-1 is nonessential for viral growth in cell culture [16].

The triplexes sit atop the MCP N-lasso triangle and are critical for capsid morphogenesis, most likely linking pentons and hexons together, directing assembly and stabilizing the structure prior to maturation. There are 320 of them in the capsid. Triplexes are heterotrimers consisting of two dimer-forming parts of the minor capsid protein (mCP), coupled with a single copy of the minor capsid binding protein (mCP-bp) [14]. The mCP of HCMV, pUL85, is a protein composed of 306 amino acids that forms dimers [9] (Table 1). The HCMV mCP-bp is encoded by the UL46 gene (Table 1).

In addition to the structural proteins, capsid assembly involves the formation of scaffold proteins for the generation of the capsid subunit and the procapsid structures. The scaffolding proteins of HCMV have been identified as gene products of the UL80a and UL80.5 open reading frames (ORFs) [17]. UL80a encodes the assembly protein precursor, which is proteolytically processed into protease and scaffold domains. The scaffold domain corresponds to the C-terminus (C-term; pUL80, aa 336 to 708) and the protease (also called assemblin, pUL80a) to the N-terminus (N-term; pUL80, aa 1 to 256). The protease is subsequently autocatalytically cleaved into two more peptides [17]. Three homologs are reported in HSV-1, VP21 (pUL26), VP24 (pUL26) and VP22a (pUL26.5) [18] (Table 1). The assembly protein, encoded by UL80.5, directly interacts with the MCP (pUL86) [19]. Importantly, the MCP–assembly protein interaction is essential for the nuclear translocation of the MCP [20]. A conserved domain in the N-term of pUL80.5 promotes self-interaction and has been suggested to lead to the generation of multimers [20]. The self-interactions together with the MCP is thought to catalyze the formation of intranuclear hexons and pentons [21]. The HSV-1 homolog UL26.5 encodes preVP22A, which interacts with the HSV-1 MCP (VP5) through its C-term 25 amino acids that remain inside A-, B- and C-capsids [22]. Specifically, 12 hydrophobic amino acids at this C-term end are critical for the interaction, suggesting that the preVP22A–VP5 interaction is a hydrophobic interaction [22].

Although the scaffold plays a central role in capsid assembly, to date, no scaffolding protein has been found within the mature capsid or the virion (Figure 2). UL80a encodes the assembly protein precursor composed of the protease (pUL80a) and the assembly protein (pUL80.5). Like HCMV, the HSV-1 pUL26 is initially extended at its N-term by the protease, and a linker and its C-term part is identical to the major form, pUL26.5 [23]. This fusion provides a convenient mechanism for incorporating the protease into the assembling procapsid. The final step of the capsid maturation in HCMV and HSV-1 involvs a proteolytic cleavage for the MCP binding domain of pUL80.5 and preVP22a, respectively [24,25]. Bioinformatics analysis has identified a protein family predicted to share the canonical herpesvirus protease fold, having conserved Ser and His residues in their active sites [23].

The capsid is also composed of a portal that occupies one of the 12 vertices of the capsid and plays a critical role for herpesvirus replication. This arrangement of 12 monomers forms a ring, allowing herpesvirus DNA to be incorporated into the preformed capsid. Portal proteins act as the docking site for the terminase–DNA complex. pUL104 is the HCMV dodecameric portal protein, with 12-fold symmetry. pUL104 directly interacts with the large terminase subunit pUL56. This interaction was shown to be essential for DNA insertion into the capsid [26]. pUL6, the HSV-1 portal protein, is one of the most studied portal proteins in herpesviruses, and its structure was recently resolved using cryo-electron microscopy [27]. Structurally, a portal monomer consists of five domains; the wing, stem, clip, β-hairpin and wall [27]. The twelve monomers are arranged such that their loop-rich wing domains form the outer periphery of the complex. The remaining stem, clip, β-hairpin and wall domains line the interior of the portal’s DNA translocation channel. The formation of a stable ring requires a putative leucine zipper, as well as disulfide bonding between subunits [28,29]. It has been shown that the portal protein pUL6 interacts with the scaffolding protein VP22a. This interaction requires a region corresponding to amino acids 143 to 151 of VP22a and is essential for viral replication, DNA cleavage/packaging and incorporation of the portal into capsids [11].