Nuclear Egress Complexes of HCMV and Other Herpesviruses: Solving the Puzzle of Sequence Coevolution, Conserved Structures and Subfamily-Spanning Binding Properties

Abstract

Herpesviruses uniquely express two essential nuclear egress-regulating proteins forming a heterodimeric nuclear egress complex (core NEC). These core NECs serve as hexameric lattice-structured platforms for capsid docking and recruit viral and cellular NEC-associated factors that jointly exert nuclear lamina as well as membrane-rearranging functions (multicomponent NEC). The regulation of nuclear egress has been profoundly analyzed for murine and human cytomegaloviruses (CMVs) on a mechanistic basis, followed by the description of core NEC crystal structures, first for HCMV, then HSV-1, PRV and EBV. Interestingly, the highly conserved structural domains of these proteins stand in contrast to a very limited sequence conservation of the key amino acids within core NEC-binding interfaces. Even more surprising, although a high functional consistency was found when regarding the basic role of NECs in nuclear egress, a clear specification was identified regarding the limited, subfamily-spanning binding properties of core NEC pairs and NEC multicomponent proteins. This review summarizes the evolving picture of the relationship between sequence coevolution, structural conservation and properties of NEC interaction, comparing HCMV to α-, β- and γ-herpesviruses. Since NECs represent substantially important elements of herpesviral replication that are considered as drug-accessible targets, their putative translational use for antiviral strategies is discussed.

Phenotypical Characterization of the Nuclear Egress of Recombinant Cytomegaloviruses Reveals Defective Replication upon ORF-UL50 Deletion but Not pUL50 Phosphosite Mutation

“…Generally, it is widely accepted knowledge that nuclear egress is determined by the functionality of herpesviral NEC proteins and represents an essential and rate-limiting step of viral replication efficiency (reviewed by [ 1 , 3 , 4 , 5 , 37 , 38 , 39 , 40 , 41 ]). A number of molecular characteristics have been identified for herpesviral core NEC proteins, among which both identical features and differences could be pointed out [ 1 , 5 , 9 , 10 , 42 ].…”

“…HCMV-induced disassembly of the nuclear lamina is mediated by a defined nuclear egress complex (NEC) that associates both viral and host proteins [ 1 ]. Two virus-encoded NEC proteins have homologs in all known herpesviruses and have been considered as functionally essential for a balanced regulation of herpesviral nuclear capsid egress [ 1 , 2 , 3 , 4 , 5 ]. This core NEC, i.e., HCMV pUL50-pUL53, provides a scaffold that is crucial for NEC multicomponent protein assembly, capsid binding and a multifaceted functionality.…”

“…The property of being phosphorylated by viral and cellular protein kinases is shared by all analyzed members of the core NEC gene family [ 1 , 3 , 9 ]. Moreover, several studies revealed strongly conserved structural features of core NECs, particularly comparing the 3D crystal structures of herpes simplex virus type 1 (HSV-1), pseudorabies virus (PrV), HCMV, murine cytomegalovirus (MCMV) and Epstein-Barr virus [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ].…”

“…Moreover, several studies revealed strongly conserved structural features of core NECs, particularly comparing the 3D crystal structures of herpes simplex virus type 1 (HSV-1), pseudorabies virus (PrV), HCMV, murine cytomegalovirus (MCMV) and Epstein-Barr virus [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Although core NEC homologs only share low to moderate degrees of sequence identity, their globular structures are remarkably conserved [ 3 ], thus comprising (i) a hook-like N-terminal heterodimerization domain in pUL53 homologs, (ii) a C-terminal α-helical binding groove in pUL50 homologs and (iii) a pUL53-specific zinc finger. Most strikingly, a common hook-into-groove principle of core NEC heterodimerization has been identified for α-, β- and γ-herpesviruses [ 1 , 3 , 5 , 9 , 10 , 14 ].…”

“…Although core NEC homologs only share low to moderate degrees of sequence identity, their globular structures are remarkably conserved [ 3 ], thus comprising (i) a hook-like N-terminal heterodimerization domain in pUL53 homologs, (ii) a C-terminal α-helical binding groove in pUL50 homologs and (iii) a pUL53-specific zinc finger. Most strikingly, a common hook-into-groove principle of core NEC heterodimerization has been identified for α-, β- and γ-herpesviruses [ 1 , 3 , 5 , 9 , 10 , 14 ]. However, limited data are available addressing the question whether core NEC homologs are strictly functionally conserved [ 1 , 17 ] or whether they may display virus-specific differences.…”