Murine CMV (cytomegalovirus) pp 89 Fragments have been derived from the amino acid sequence of murine cytomegalovirus protein pp 89, an immediate-early phase regulatory protein of murine cytomegalovirus.
Cytomegalovirus (CMV) is an ubiquitous human pathogen that is responsible for congenital malformations. Within the 235-kilobase-pair MCMV DNA, gene expression during the IE (immediate-early) phase is limited to a region of 12 kbp, which includes the transcription units IEl, IE2, and IE3. Gene IEl contained in transcription unit iel encodes the major MCMV IE protein, the non-structural regulatory phosphoprotein pp89. The murine cytomegalovirus protein pp89 is expressed in the immediate-early phase of the viral replication cycle and located mainly in the nucleus of infected cells 1,2,3.
The amino acid sequence of the Murine CMV IE1 protein contains a region of homology to histone H2B and a highly acidic carboxy-terminal region. In vitro studies have revealed that pp89 is eluted from chromatin at low salt concentration, but binds strongly to isolated histones. Murine CMV pp89 does not bind to condensed chromatin and that neither the region of homology to histone H2B nor the acidic region are essential for histone binding but interacts with both DNA associated and isolated histones in vitro. Analysis of pp89 deletion mutants and of fragments generated by cleavage at pH 2.5 revealed that the regions responsible for association with histone are located between amino acids 71 and 415, and are not identical with the domain that shows homology to histone H2B or the highly acidic carboxy-terminal region. A potential gene-activating role of pp89 is the high affinity for isolated histones and the low affinity for DNA associated histones 4.
Mode of Action
Virus-specific, major histocompatibility complex class I-restricted cytolytic T lymphocytes (CTL) recognize pp89. Vaccination with a -recombinant vaccinia virus, MCMV-IEI-VAC, expressing pp89 as the only MCMV protein, induces protective immunity. A single domain of pp89 is relevant for recognition by CD8+ T lymphocytes in BALB/c mice and led to the identification of an epitope 19 amino acids in length within this domain. The domain is entirely encoded by the fourth exon of gene IEI and contains the sequences essential for both immunogenicity and antigenicity of pp89 5,6.
Activation of E gene promoters- Thorough functional analysis of the homologous gene complex, IE1/IE2, in human CMV has revealed that IE1 products activate the IE1 gene promoter, whereas IE2 products have been found to be essential for the activation of E (early) gene promoters. Histone-binding domains of pp89 support the establishment of stable transcription complexes during nucleosomal organization by direct interaction with histones. In addition, the highly acidic C-terminal sequence of pp89 is free to exert functions 4,7.
Murine CMV protection- IE protein pp89 is the only MCMV gene product that can protect against a lethal challenge infection with Murine CMV. Protection is mediated by pp89-specific CD8+ T lymphocyte. The epitope comprises amino acids 161 to 179, contains a proposed pattern at residues 170 to 174, and is the first described to be presented by the major histocompatibility complex class I molecule. This peptide contains three prolines, an amino acid seldom found in a-helices and in epitopes recognized by T lymphocytes 8.
DHFR stimulation- Murine CMV infection of quiescent NIH 3T3 cells markedly stimulates transcription, expression and activity of the cellular dihydrofolate reductase (DHFR), a key enzyme in the synthesis of DNA precursors. DHFR stimulation by Murine CMV is sensitive to UV irradiation and seems to depend on expression of the viral immediate-early protein pp89 9,10.
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