Epstein-Barr virus gH is essential for penetration of B cells but also plays a role in attachment of virus to epithelial cells

Entry of Epstein-Barr virus (EBV) into B cells is initiated by attachment of glycoprotein gp350 to the complement receptor type 2 (CR2). A complex of three glycoproteins, gH, gL, and gp42, is subsequently required for penetration. Gp42 binds to HLA class II, which functions as an entry mediator or coreceptor and, by analogy with other herpesviruses, gH is then thought to be involved virus-cell fusion. However, entry of virus into epithelial cells is thought to be different. It can be initiated by attachment by an unknown glycoprotein in the absence of CR2. There is no interaction between gp42 and HLA class II and instead a distinct complex of only the two glycoproteins gH and gL interacts with a novel entry mediator. Again, by analogy with other viruses gH is thought to be critical to fusion. To investigate further the different roles of gH in infection of the two cell types and to examine its influence on the assembly of the gH-gL-gp42 complex, we constructed two viruses, one in which the gH open reading frame was interrupted by a cassette expressing a neomycin resistance gene and the gene for green fluorescent protein and one as a control in which the neighboring nonessential thymidine kinase gene was interrupted with the same cassette. Virus lacking gH exited from cells normally, although loss of gH resulted in rapid turnover of gL and gp42 as well. The virus bound normally to B lymphocytes but could not infect them unless cells and bound virus were treated with polyethylene glycol to induce fusion. In contrast, virus that lacked the gH complex was impaired in attachment to epithelial cells and the effects of monoclonal antibodies to gH implied that this resulted from loss of gH rather than other members of the complex. These results suggest a role for gH in both attachment and penetration into epithelial cells.

Epstein-barr virus regulates c-MYC, apoptosis, and tumorigenicity in Burkitt lymphoma

Loss of the Epstein-Barr virus (EBV) genome from Akata Burkitt lymphoma (BL) cells is coincident with a loss of malignant phenotype, despite the fact that Akata and other EBV-positive BL cells express a restricted set of EBV gene products (type I latency) that are not known to overtly affect cell growth. Here we demonstrate that reestablishment of type I latency in EBV-negative Akata cells restores tumorigenicity and that tumorigenic potential correlates with an increased resistance to apoptosis under growth-limiting conditions. The antiapoptotic effect of EBV was associated with a higher level of Bcl-2 expression and an EBV-dependent decrease in steady-state levels of c-MYC protein. Although the EBV EBNA-1 protein is expressed in all EBV-associated tumors and is reported to have oncogenic potential, enforced expression of EBNA-1 alone in EBV-negative Akata cells failed to restore tumorigenicity or EBV-dependent down-regulation of c-MYC. These data provide direct evidence that EBV contributes to the tumorigenic potential of Burkitt lymphoma and suggest a novel model whereby a restricted latency program of EBV promotes B-cell survival, and thus virus persistence within an immune host, by selectively targeting the expression of c-MYC.

Epstein-Barr virus recombinant lacking expression of glycoprotein gp150 infects B cells normally but is enhanced for infection of epithelial cells

Glycoprotein gp150 is a highly glycosylated protein encoded by the BDLF3 open reading frame of Epstein-Barr virus (EBV). It does not have a homolog in the alpha- and betaherpesviruses, and its function is not known. To determine whether the protein is essential for replication of EBV in vitro, a recombinant virus which lacked its expression was made. The recombinant virus had no defects in assembly, egress, binding, or infectivity for B cells or epithelial cells. Infection of epithelial cells was, however, enhanced. The glycoprotein was sensitive to digestion with a glycoprotease that digests sialomucins, but no adhesion to cells that express selectins that bind to sialomucin ligands could be detected.

Role of heme-hemopexin in human T-lymphocyte proliferation

Heme-hemopexin supports and stimulates proliferation of human acute T-lymphoblastic (MOLT-3) cells, suggesting the participation of heme in cell growth and division. MOLT-3 cells express approximately 58,000 hemopexin receptors per cell (apparent Kd 20 nM), of which about 20% are on the cell surface. Binding is dose- and temperature-dependent, and growth in serum-free IMDM medium is stimulated by 100-1000 nM heme-hemopexin, consistent with the high affinity of the receptor for hemopexin, and maximal growth is seen in response to 500 nM complex. Growth was similar in defined minimal medium supplemented with either low concentrations of heme-hemopexin or iron-transferrin, and either of these complexes were about 80% as effective as a serum supplement. Heme-hemopexin, but not apo-hemopexin, reversed the growth inhibition caused by desferrioxamine showing that heme-iron derived from heme catabolism is used for cell growth. Cobalt-protoporphyrin (CoPP)-hemopexin, which binds to the receptor but is not transported intracellularly [Smith et al., (1993) J. Biol. Chem. 268, 7365], also stimulated cell proliferation in serum-free IMDM but did not "rescue" the cells from desferrioxamine. Furthermore, CoPP-hemopexin effectively competed for the hemopexin receptor with heme-hemopexin and diminished its growth stimulatory effects. In addition, protein kinase C (PKC) is translocated to the plasma membrane within 5 min after heme-hemopexin is added to the medium, reaches maximum activity within 5-10 min, and declines to unstimulated levels by 30 min. Heme-hemopexin and CoPP-hemopexin both augmented MOLT-3 cell growth stimulated by serum. Thus, heme-hemopexin not only functions as an iron source for T-cells but occupancy of the hemopexin receptor itself triggers signaling pathway(s) involved in the regulation of cell growth. The stimulation of growth of human T-lymphocytes by heme-hemopexin is likely to be a physiologically relevant mechanism at sites of injury, infection, and inflammation.

Histones of Chlamydomonas reinhardtii. Synthesis, acetylation, and methylation

Histones of the green alga Chlamydomonas reinhardtii were prepared by a new method and fractionated by reversed-phase high-performance liquid chromatography. Acid-urea-Triton gel analysis and tritiated acetate labeling demonstrated high levels of steady-state acetylation for the single histone H3 protein, in contrast to low levels on histones H4 and H2B. Twenty percent of histone H3 is subject to dynamic acetylation with, on average, three acetylated lysine residues per protein molecule. Histone synthesis in light-dark-synchronized cultures was biphasic with pattern differences between two histone H1 variants, between two H2A variants, and between H2B and ubiquitinated H2B. Automated protein sequence analysis of histone H3 demonstrated a site-specific pattern of steady-state acetylation between 7 and 17% at five of the six amino-terminal lysines and of monomethylation between 5 and 81% at five of the eight amino-terminal lysines in a pattern that may limit dynamic acetylation. An algal histone H3 sequence was confirmed by protein sequencing with a single threonine as residue 28 instead of the serine28-alanine29 sequence, present in all other known plant and animal H3 histones.