Nuclear translocation of mutagenized forms of human cytomegalovirus glycoprotein B (gpUL55)

Conserved cytoplasmic domain sequences mediate the ER export of VZV, HSV-1, and HCMV gB

Glycoprotein B (gB) is conserved among the herpesviruses and participates in both virus entry and cell-cell spread. The ER export of VZV gB is mediated by two cytoplasmic domain regions, aa 818-826, which contains a YXXphi motif, and the C-terminal 17 aa. The current study examines whether related sequences in the cytoplasmic domains of HSV-1 and HCMV gB similarly influence the ER export of their gB homologs. Directed mutations were introduced into the cytoplasmic domains of HSV-1 and HCMV gB, and the efficiencies with which the mutated proteins acquired Golgi-dependent modifications were determined. Sequences homologous to VZV gB aa 818-826 were required for normal ER export of both HSV-1 gB and HCMV gB. However, the C-terminal regions of HSV-1 and HCMV gB had no impact on ER export. Therefore, alpha- and betaherpesvirus gB homologs share conserved ER export signals, but species-specific differences in the ER export of gB also exist.

A three-residue signal confers localization of a reporter protein in the inner nuclear membrane

By alignment to the carboxy-terminal-deduced aa sequence of human cytomegalovirus glycoprotein B (gB), conserved hexameric aa motifs with putative function for localization in the inner nuclear membrane (INM) were identified in the nucleoplasmic tails of herpes simplex virus type 1 gB and of the cellular lamin B receptor. Fusion of the respective hexamers to CD8 as a reporter redirected transport of the chimeras into the INM, suggesting their functioning as modular signal motifs. Consecutive experiments showed that the three-residue motif RxR represents a consensus sequence which is sufficient for localization of the CD8 reporter in the INM.

Multimerization potential of the cytoplasmic domain of the human cytomegalovirus glycoprotein B

In the present study the coding sequence of the cytoplasmic tail of the human cytomegalovirus glycoprotein B (gB) was expressed. The secondary structure of the purified recombinant protein was analyzed by circular dichroism, and the quaternary structure was investigated by gel permeation chromatography, and electron microscopy. Our data indicate that the cytoplasmic gB domain contains alpha-helix structures and assembles into tetramers, suggesting that the authentic gB may represent a homotetramer.

Identification of a novel signal sequence that targets transmembrane proteins to the nuclear envelope inner membrane

Herpesvirus maturation requires translocation of glycoprotein B homologue from the endoplasmic reticulum to the inner nuclear membrane. Glycoprotein B of human cytomegalovirus was used in this context as a model protein. To identify a specific signal sequence within human cytomegalovirus glycoprotein B acting in a modular fashion, coding sequences were recombined with reporter proteins. Immunofluorescence and cell fractionation demonstrated that a short sequence element within the cytoplasmic tail of human cytomegalovirus glycoprotein B was sufficient to translocate the membrane protein CD8 to the inner nuclear membrane. This carboxyl-terminal sequence had no detectable nuclear localization signal activity for soluble beta-Galactosidase and could not be substituted by the nuclear localization signal of SV40 T antigen. For glycoprotein B of herpes simplex virus, a carboxyl-terminal element with comparable properties was found. Further experiments showed that the amino acid sequence DRLRHR of human cytomegalovirus glycoprotein B (amino acids 885-890) was sufficient for nuclear envelope translocation. Single residue mutations revealed that the arginine residues in positions 4 and 6 of the DRLRHR sequence were essential for its function. These results support the view that transmembrane protein transport to the inner nuclear membrane is controlled by a mechanism different from that of soluble proteins.

Constitutive expression of human cytomegalovirus (HCMV) glycoprotein gpUL75 (gH) in astrocytoma cells: a study of the specific humoral immune response

The humoral immune response to gpUL75 (gH) was determined in different groups of human cytomegalovirus (HCMV) infected subjects using a full-length glycoprotein constitutively expressed in an astrocytoma cell line. The recombinant molecule consisted of two distinct isoforms resembling the authentic protein of infected cells. Separated from the interactions of other viral gene products gH failed to form an oligomeric complex, thus exhibiting exclusively epitopes present on the monomer. Ninety five percent of serum samples from latently-infected healthy adults revealed the presence of gH-specific IgG. Moreover, examination of sequential sera from immunocompromised and immunocompetent individuals undergoing active HCMV infection demonstrated that antibodies to gH occurred in most cases simultaneously with those to the abundant surface antigen gpUL55 (gB) and at similar titres. Appearance of this response was correlated with a considerable increase of the virus-neutralizing activity and most likely associated with restriction of viral dissemination during subsequent viremic episodes. Together, these results suggest that glycoprotein H of HCMV is like gB, a highly immunogenic component of the infectious particle.

Steady-state plasma membrane expression of human cytomegalovirus gB is determined by the phosphorylation state of Ser900

Human cytomegalovirus (HCMV) infection of an astrocytoma cell line (U373) or human fibroblast (HF) cells results in a differential cell distribution of the major envelope glycoprotein gB (UL55). This 906-amino-acid type I glycoprotein contains an extracellular domain with a signal sequence, a transmembrane domain, and a 135-amino-acid cytoplasmic tail with a consensus casein kinase II (CKII) site located at Ser900. Since phosphorylation of proteins in the secretory pathway is an important determinant of intracellular trafficking, the state of gB phosphorylation in U373 and HF cells was examined. Analysis of cells expressing wild-type gB and gB with site-specific mutations indicated that the glycoprotein was equally phosphorylated at a single site, Ser900, in both U373 and HF cells. To assess the effect of charge on gB surface expression in U373 cells, Ser900 was replaced with an aspartate (Asp) or alanine (Ala) residue to mimic the phosphorylated and nonphosphorylated states, respectively. Expression of the Asp but not the Ala gB mutation resulted in an increase in the steady-state expression of gB at the plasma membrane (PM) in U373 cells. In addition, treatment of U373 cells with the phosphatase inhibitor tautomycin resulted in the accumulation of gB at the PM. Interestingly, the addition of a charge at Ser900 trapped gB in a low-level cycling pathway at the PM, preventing trafficking of the protein to the trans-Golgi network or other intracellular compartments. Therefore, these results suggest that a tautomycin-sensitive phosphatase regulates cell-specific PM retrieval of gB to intracellular compartments.