Date Published: May 19, 2017
Publisher: Public Library of Science
Author(s): An Li, Guangwen Lu, Jianxun Qi, Lili Wu, Kegong Tian, Tingrong Luo, Yi Shi, Jinghua Yan, George F. Gao, Z. Hong Zhou.
An early and yet indispensable step in the alphaherpesvirus infection is the engagement of host receptors by the viral envelope glycoprotein D (gD). Of the thus-far identified gD receptors, nectin-1 is likely the most effective in terms of its wide usage by multiple alphaherpesviruses for cell entry. The molecular basis of nectin-1 recognition by the gD protein is therefore an interesting scientific question in the alphaherpesvirus field. Previous studies focused on the herpes simplex virus (HSV) of the Simplexvirus genus, for which both the free gD structure and the gD/nectin-1 complex structure were reported at high resolutions. The structural and functional features of other alphaherpesviral gDs, however, remain poorly characterized. In the current study, we systematically studied the characteristics of nectin-1 binding by the gD of a Varicellovirus genus member, the pseudorabies virus (PRV). We first showed that PRV infects host cells via both human and swine nectin-1, and that its gD exhibits similar binding affinities for nectin-1 of the two species. Furthermore, we demonstrated that removal of the PRV gD membrane-proximal residues could significantly increase its affinity for the receptor binding. The structures of PRV gD in the free and the nectin-1-bound states were then solved, revealing a similar overall 3D fold as well as a homologous nectin-1 binding mode to its HSV counterpart. However, several unique features were observed at the binding interface of PRV gD, enabling the viral ligand to utilize different gD residues (from those of HSV) for nectin-1 engagement. These observed binding characteristics were further verified by the mutagenesis study using the key-residue mutants of nectin-1. The structural and functional data obtained in this study, therefore, provide the basis of receptor recognition by PRV gD.
There are three major subfamilies, Alpha-, Beta- and Gamma-herpesvirinae, in the Herpesviridae family . The three subfamilies differ in their host range capacities. In contrast to beta and gamma herpesviruses which exhibit restricted or limited cell-type tropism, the alphaherpesviruses have a much broader host range and are able to infect a wide variety of cell types [2, 3]. For example, the representative alphaherpesvirus, herpes simplex virus (HSV), shows low species specificity and is able to infect human and non-human cells alike . The capability of HSV to infect most human cell types is recognized as an important contributing factor to its high prevalence in the world populations . Pseudorabies virus (PRV), another member of the Alphaherpesvirinae subfamily, is reported to infect both farming (e. g. pigs, sheep, etc) and pet (e. g. cats) animals . Herpes B virus, an alphaherpesvirus that causes mild or asymptomatic infections in macaques, can cross the species barriers and lead to fatal diseases in humans .
Of the thus-far identified alphaherpesviral gD receptors, nectin-1 is likely the most effective in terms of its wide usage by different viruses. HSV-1, HSV-2, PRV and BHV-1 are all reported to utilize nectin-1 for cell entry [8, 12, 13, 30, 31]. The molecular basis of nectin-1 recognition by the envelope gD proteins of these viruses, therefore, is an interesting but yet an unresolved issue. Previous studies focused on HSV, a member of the Simplexvirus genus in the Alphaherpesvirinae subfamily. The structures of both the free HSV gD and its complex with nectin-1 are reported at high resolutions [6, 21, 22, 25]. Nevertheless, the structural and functional features of other alphaherpesviral gDs remains poorly understood. In this study, we have reported the first structure of gD derived from a Varicellovirus member of the alphaherpesviruses, the PRV. Despite of its low sequence identity to the HSV homologs (~ 22%), PRV gD reserves the canonical gD features, including an IgV-like core with a kinked C” strand and the surface-exposed N- and C-terminal extensions. We further solved the complex structure of PRV gD bound with SW-nectin-1, which revealed a similar nectin-1 binding mode as observed for HSV gD. Nevertheless, several unique features at the PRV-gD/nectin-1 binding interface (e. g. a bulged α2′ helix of PRV gD interacts with an adjusted CC’ loop in nectin-1, an about 3.5-Å shift for PRV gD (relative to HSV gD) when bound to the receptor) suffice the PRV ligand to recognize nectin-1 using quite different gD residues from those of the HSV homologs. These structural observations therefore provide a systematic view on the receptor binding mechanism of a second alphaherpesvirus and yet the first in the Varicellovirus genus.