title: Mutations in Pseudorabies Virus Glycoproteins gB, gD, and gH Functionally Compensate for the Absence of gL.
abstract: Entry of herpesviruses depends on the combined action of viral glycoprotein B (gB) and the heterodimeric gH/gL complex, which are activated by binding of the virion to specific cellular receptors. While gB carries signatures of a bona fide fusion protein, efficient membrane fusion requires gH/gL. However, although gB and gH/gL are essential for entry, the alphaherpesvirus pseudorabies virus (PrV) is capable of limited cell-to-cell spread in the absence of gL. To understand gH/gL function in more detail, the limited spread of PrV-ΔgL was used for reversion analyses by serial cell culture passages. In a first experiment, an infectious gL-negative mutant in which gL function was replaced by generation of a gD-gH hybrid protein was isolated (B. G. Klupp and T. C. Mettenleiter, J Virol 73:3014-3022, 1999). In a second, independent experiment PrV-ΔgLPassB4.1, which also replicated productively without gL, was isolated. Sequence analysis revealed mutations in gH but also in gB and gD. In a transfection-based fusion assay, two amino acid substitutions in the N-terminal part of gH(B4.1) (L(70)P and W(103)R) were found to be sufficient to compensate for lack of gL, while mutations present in gB(B4.1) enhanced fusogenicity. Coexpression of gB(B4.1) with the homologous gH(B4.1) resulted in strongly increased syncytium formation, which was further augmented by truncation of the gB(B4.1) C-terminal 29 amino acids. Nevertheless, gH was still required for membrane fusion. Surprisingly, coexpression of gD(B4.1) blocked syncytium formation in the fusion assays, which could be attributed to a V(106)A substitution within the ectodomain of gD(B4.1).
  
  
  OBJECTIVE
  In contrast to many other enveloped viruses, herpesviruses rely on the concerted action of four viral glycoproteins for membrane fusion during infectious entry. Although the highly conserved gB shows signatures of a fusion protein, for fusion induction it requires the gH/gL complex, whose role is still elusive. Here we demonstrated fusion activation by gH in the absence of gL after reversion analysis of gL-deleted pseudorabies virus. This gL-independent fusion activity depended on single amino acid exchanges affecting the gL-binding domain in gH, increasing fusogenicity in gB and allowing negative fusion regulation by gD. Thus, our results provide novel information on the interplay in the fusion machinery of herpesviruses.
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Christina
authors.initials: C
authors.lastname: Schröter
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Melina
authors.initials: M
authors.lastname: Vallbracht
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Altenschmidt
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Kargoll
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Fuchs
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany.
authors.forename: Barbara G
authors.initials: BG
authors.lastname: Klupp
authors.affiliation: Institute of Molecular Virology and Cell Biology, Friedrich-Loeffler-Institut, Greifswald-Insel Riems, Germany thomas.mettenleiter@fli.bund.de.
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Mettenleiter
mesh_terms: D000818:Animals; D002460:Cell Line; D004252:DNA Mutational Analysis; D004279:DNA, Viral / Q000737:chemistry / Q000235:genetics; D017353:Gene Deletion; D011558:Herpesvirus 1, Suid / Q000235:genetics / Q000502:physiology*; D011817:Rabbits; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D012692:Serial Passage; D013489:Suppression, Genetic; D014759:Viral Envelope Proteins / Q000235:genetics* / Q000378:metabolism*; D053586:Virus Internalization
chemical_list: D004279:DNA, Viral; D014759:Viral Envelope Proteins; C042966:pseudorabies virus glycoproteins