Divergent Evolution of Nuclear Localization Signal Sequences in Herpesvirus Terminase Subunits

Abstract

The tripartite terminase complex of herpesviruses assembles in the cytoplasm of infected cells and exploits the host nuclear import machinery to gain access to the nucleus, where capsid assembly and genome-packaging occur. Here we analyzed the structure and conservation of nuclear localization signal (NLS) sequences previously identified in herpes simplex virus 1 (HSV-1) large terminase and human cytomegalovirus (HCMV) small terminase. We found a monopartite NLS at the N terminus of large terminase, flanking the ATPase domain, that is conserved only in ␣-herpesviruses. In contrast, small terminase exposes a classical NLS at the far C terminus of its helical structure that is conserved only in two genera of the ␤-subfamily and absent in ␣-and ␥-herpesviruses. In addition, we predicted a classical NLS in the third terminase subunit that is partially conserved among herpesviruses. Bioinformatic analysis revealed that both location and potency of NLSs in terminase subunits evolved more rapidly than the rest of the amino acid sequence despite the selective pressure to keep terminase gene products active and localized in the nucleus. We propose that swapping NLSs among terminase subunits is a regulatory mechanism that allows different herpesviruses to regulate the kinetics of terminase nuclear import, reflecting a mechanism of virus:host adaptation.

Mutual Interplay between the Human Cytomegalovirus Terminase Subunits pUL51, pUL56, and pUL89 Promotes Terminase Complex Formation

“…Taking all these results together, we concluded that correct nuclear localization of both pUL51 and pUL89 requires the concurrent presence of all three terminase subunits. Conversely, pUL56 is located in the nucleus when expressed alone or in the absence of either pUL51 or pUL89, which is in accordance with the presence of a nuclear localization signal in the UL56 protein (52,53).…”

“…By applying more sensitive microscope settings, we now saw that in cells adenofected with the UL51 null genome, pUL56 was nuclear, whereas pUL89 was restricted to the cytoplasm. In general, pUL89, which does not contain a nuclear localization signal (NLS) according to in silico analyses (53), was cytoplasmic when one of the other terminase components was missing. pUL51, for which no classical NLS was predicted (53), was distributed throughout the cell in the absence of pUL56 or pUL89.…”

“…In general, pUL89, which does not contain a nuclear localization signal (NLS) according to in silico analyses (53), was cytoplasmic when one of the other terminase components was missing. pUL51, for which no classical NLS was predicted (53), was distributed throughout the cell in the absence of pUL56 or pUL89. This can be explained by the small size of pUL51, allowing diffusion through the nuclear pore complex.…”

The C-terminal 18 Amino Acid Region of Dengue Virus NS5 Regulates its Subcellular Localization and Contains a Conserved Arginine Residue Essential for Infectious Virus Production

“…Overall, the affinity of DENV2 and 3 NS5 for importin by in vitro assay are similar while the affinity of DENV1 and 4 NS5 are weak. The differences in affinity measurement are within the expected range noted for well characterised targets such as SV40 T-ag NLS which can vary from 10–1000 nM depending on the methodolgy that is used [ 38 , 39 ].…”

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

“…A characteristic of HSVs is that capsids are assembled within the nucleus of infected cells (Morgan et al, 1954 ). Viral capsid proteins that are synthetized in the cytoplasm are translocated to the nucleus thanks to nuclear localization sequences (NLS) that signal these proteins to this compartment where viral genome replication takes place (Figure 3 , process 7 and Table 1 ) (Abaitua et al, 2012 ; Sankhala et al, 2016 ). HSV capsids are mainly composed of the major capsid protein VP5 and also by other less abundant viral proteins, such as VP19C, VP23, and VP26 (Thomsen et al, 1994 ; Homa and Brown, 1997 ; Kobayashi et al, 2017 ).…”