Transcription of BamHI-A region of the EBV genome in NPC tissues and B cells

Analysis by Northern blotting and sequencing of cDNA clones from a transcription library of a tumor biopsy from a nasopharyngeal carcinoma (NPC) patient showed that the BamHI-A region of the Epstein-Barr virus genome is abundantly and regularly transcribed in tumor tissues from NPC patients. The transcription occurred in a rightward direction terminating between coordinates 160,965 and 160,995, where two polyadenylation sites are located. Rightward transcription of this region also occurred in B lymphoid cells harboring the viral genome, albeit at a lower level than in the tumor tissues. Differential splicing yields a family of related transcripts displaying at least four splicing patterns. Different promoters may be utilized, further contributing to the diversity of this family of transcripts. A 2.8-kb unspliced transcript present in B95-8 cells was probably initiated from a TATA box located in position 158,204, while the other transcripts may utilize other promoters localized to other regions. All the transcripts encompass a putative open reading frame, BARFO, which is predicted to encode a basic protein of about 20 kDa. It shares 40% colinear amino acid sequence homology with the DNA binding region of a transcription factor, ICP4, specified by herpes simplex virus.

Alternative splicing determines the carboxy terminus of the Epstein-Barr virus nuclear antigen 5 species expressed in the Burkitt's lymphoma cell line Daudi

The Daudi strain of Epstein-Barr virus (EBV) possesses a genomic deletion, relative to the B95-8 EBV prototype, that removes the entire Epstein-Barr virus nuclear antigen 2 (EBNA2) open reading frame (ORF) and the sequences encoding the carboxy terminus of EBNA5. Immunoblot analysis carried out in this study indicates that two species of EBNA5 (31K and 37K) are expressed in Daudi cells. Nucleotide sequence analysis of Daudi cDNA clones has confirmed that, as a consequence of the genomic deletion, exons usually appearing further downstream in EBNA messages (exons U or HF) are spliced directly onto the truncated EBNA5 ORF. Furthermore, the use of alternative splicing suggests that the two EBNA5 species expressed in Daudi cells possess different carboxy termini.

Expression of a family of complementary-strand transcripts in Epstein-Barr virus-infected cells

A major family of polyadenylylated cytoplasmic transcripts are expressed from the BamHI A-I region of the Epstein-Barr virus genome, off the strand complementary to that encoding several functions associated with viral replication and the lytic cycle, including the DNA polymerase (BALF-5). These complementary-strand transcripts (the main one is about 4.8 kilobases long), expressed in all cell types associated with Epstein-Barr virus, are present at high levels in nasopharyngeal carcinoma tumors. Sequence analysis of clones that correspond to spliced transcripts in a cDNA library from such a tumor, C15, generates a profile of the main complementary mRNA. It contains at least three AUG-initiated open reading frames, the largest of which could be translated to give a polypeptide of about 20 kDa. Evidence from several types of experiments suggests that conditions which support the up (or down) regulation of transcriptional expression from one viral DNA strand within the relevant region of the genome produce the opposite effect on transcripts from the other strand. The capacity for interference between complementary Epstein-Barr viral transcripts offers a mechanism for control of gene expression that may be related to maintenance of viral latency.

Stimulation of cell division and fibroblast focus formation by antisense repression of retinoblastoma protein synthesis

Circumstantial evidence supports a role for the retinoblastoma susceptibility gene, Rb-1, in the maintenance of normal cell growth, in that loss of its function results in abnormal growth and malignancy. Here we report that a high rate of mitosis and efficient dense focus formation in human embryonic lung fibroblasts (HEL cells) is induced by antisense oligonucleotide-directed inhibition of synthesis of p105-Rb, the product of the Rb-1 gene. mRNA specific for p105-Rb is truncated at the site of base pairing with the antisense oligonucleotide, and no synthesis of p105-Rb is observed. The rate of mitosis is considerably increased and the frequency of dense focus formation is extremely high in treated cells. However, although phosphothioate oligodeoxyribonucleotides taken up by the cells remain stable for at least 4 weeks, the recipient cells do not become immortal; nor are they able to induce tumor formation in nude mice. Thus, loss of Rb-1 function is not sufficient per se to allow malignant transformation.

Transcription of the Epstein-Barr virus gene EBNA-1 from different promoters in nasopharyngeal carcinoma and B-lymphoblastoid cells

Transcriptional expression of the Epstein-Barr virus (EBV) genome has been shown to differ markedly between nasopharyngeal carcinoma (NPC) cells and latent B-cell lines, with a more limited pattern of gene expression seen in NPC. EBNA-1 is the only nuclear antigen so far detected in both NPC and Burkitt's lymphoma cells. We found previously that in a human NPC tumor passaged in nude mice, designated C15, the EBNA-1 mRNA contained a novel splice site in the BamHI Q region of EBV which had not previously been described for B-cell lines. This lies within a region of the EBV genome to which EBNA-1 binds. Here, we further characterize the 5' region of EBNA-1 transcripts and identify two splicing patterns in C15 cells; we show that they are derived from a common promoter region in the BamHI F region of the viral genome. We also demonstrate that this region can function to initiate transcription of the chloramphenicol acetyltransferase gene in epithelial cells and that the promoter region is only partially methylated at CpG sites in the tumor. In contrast, a B-lymphoblastoid cell line derived from C15 uses a conventional promoter in BamHI-C/W for expression of EBNA-1.

Characterization of the DNA polymerase gene of human herpesvirus 6

The construction of a recombinant bacteriophage lambda library containing overlapping clones covering 155 kbp of the 161-kbp genome of the Ugandan U1102 isolate of human herpesvirus 6 (HHV-6) is described. The use of degenerate-primer polymerase chain reaction allowed the isolation of a DNA probe for the DNA polymerase gene of HHV-6, which was subsequently used to isolate and position the pol gene on the physical map of the viral genome. A 4.4-kbp EcoRI DNA restriction fragment containing the pol gene was isolated and sequenced. The open reading frames flanking the pol gene code for the HHV-6 glycoprotein B gene and the human cytomegalovirus UL53 homolog. This arrangement is different from that seen in the alpha and gamma herpesvirus families, lending further support to the notion that HHV-6 is a member of the beta herpesvirus group.

Immortalization of human fetal sinusoidal liver cells by polyoma virus large T antigen

Fetal sinusoidal liver cells were isolated from human liver explant cultures and transfected with pCMVLT, a plasmid containing the immediate early promotor of cytomegalovirus (CMV) and the large tumor antigen (LT) coding part of the polyoma virus (py) genome. Whereas nontransfected cells stopped proliferating after 4 weeks, the transfected sinusoidal cells were stimulated to divide more quickly without changes in their morphology. Up to now, cells have been permanently cultured for more than 18 months and passaged over 130 times, corresponding to around 400 generations. This allows them to be regarded as "immortalized" cells. The presence of LT protein in the cells has been documented by means of immunoprecipitation and immunofluorescence. Expression of the v. Willebrandt factor VIII was the main criterion for classifying the cell population as endothelial cells. The presence of cytokeratins 7, 8, and 18 in these cells underlines their close ontogenic and functional relationship to mesothelial cells. Sinusoidal endothelial cells (SECs) synthesize vimentin and the typical extracellular matrix components collagen IV and fibronectin, but are negative for laminin and entactin. We used immortalized SEC's in co-culture experiments with fresh fetal human hepatocytes and adult mouse hepatocytes. They promoted survival of both types of hepatocytes over a period of 8-10 weeks. Control human fetal liver explant culture cells survived for only 3-4 weeks, whereas control adult mouse liver cells retained vitality for 8-10 days only.

Differential expression of Epstein Barr viral transcripts for two proteins (TP1 and LMP) in lymphocyte and epithelial cells

Studies presented here show that some functions of the human herpesvirus, EBV, may be transcriptionally differentially expressed in two cell types which carry the same (C15) isolate of this virus. Of the 'latent' viral functions investigated, only one (TP2) of the episomally-specific genes that encode terminal proteins (TP1 and TP2) is found to be expressed in the C15 epithelial cell tumour environment, whereas both are transcribed--as different, but related, messengers--in a B-cell line generated with virus from the C15 tumour. The other gene investigated is that for latent membrane protein (LMP), which is found in the same region of the EBV genome but on the opposite strand. This gene, apparently transcriptionally silent in B-cell (Burkitt's) lymphomas, is expressed in the C15 epithelial tumour, as well as in other nasopharyngeal carcinomas investigated. Promoter usage in the carcinomas and B-cells appears, in some cases at least, to be cell-type specific. Expression may also be governed by methylation since a chromosomally silent region in the carcinoma (that encompassing TP1) is highly methylated on CpG residues, whereas the active region (encoding TP2 and LMP) is virtually free of such methylation. Our data suggest that there may be selective transcriptional regulation of EBV genes in the two types of cells investigated. Thus, it may be unnecessary to invoke different virus genotypes to account for the two distinct malignancies--Burkitt's lymphoma and nasopharyngeal carcinoma--associated with EBV.

Epstein-Barr virus receptors but not viral DNA are present in normal and malignant oral epithelium

The presence and distribution of Epstein-Barr Virus receptors (EBVR's) on a range of normal (n = 18), dysplastic (n = 10) and malignant (n = 20) oral mucosa were studied by immunocytochemical methods using the monoclonal antibodies (MAb's) HB5 and B2. EBVR's were demonstrated as membrane staining of the spinous layers of normal non- and parakeratinized epithelium, indicating that EBVR's are differentiation-linked. This distribution was retained in dysplastic epithelium. Tissue from oral squamous cell carcinomas (SCC's) showed variable reactivity of only a few cells scattered randomly within the samples. Furthermore, a sensitive in situ hybridization (ISH) technique was used to determine if Epstein-Barr virus (EBV) was present in normal (n = 15) and oral squamous cell carcinoma tissue (n = 20). No EBV DNA was demonstrated within either normal or malignant epithelium, suggesting that the virus does not persist in normal oral stratified squamous epithelium nor is there any evidence for a role in oral carcinogenesis.

Cellular immortalization--an essential step or merely a risk factor in DNA virus-induced transformation?

Different activities of DNA viral gene products seem to be involved in the immortalization process, even in cases where continued presence of the viral genome does not seem to be required for the maintenance of the immortalized state of a cell. Immortalization, does not appear to represent a single event as implied earlier and several studies have shown that the process can be reversible. Polyomavirus large T antigen and HPV E7 (or E6 + E7) seem to possess all the activities required in vitro for immortalization of human cells, whereas one of the required activities--that defined in the two-step model as a rare mutagenic event which occurs during cellular crisis--is weaker in SV40 large T antigen and E1A. Viral functions that can activate PCNA expression (or repress Rb1 expression) have to be considered as pivotal activities in immortalization. Finally, the growth factor independence characterizing many immortalized cells could be a result of growth factor-like activities intrinsic to the viral proteins or could reflect their ability to induce autocrine growth mechanisms. These statements all relate to the first aspect of our initial hypothesis concerning cellular immortalization and in general substantiate it. Is immortalization an essential step in malignant transformation? There seems no a priori reason that transformation or tumorigenesis should depend upon cellular immortalization. Notably, many tumors appear to be mortal in culture. Growth factor independence or activation of DNA replication--essential features of immortalization--are probably of little importance for tumors in vivo where a crucial environment is supplied by the surrounding cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Myristylated polyomavirus VP2: role in the life cycle of the virus

The double-stranded genome of the small DNA tumor virus, polyomavirus, is enclosed in a capsid composed of a major protein, VP1, which associates as pentameric capsomeres into an icosahedral structure, and two minor proteins, VP2 and VP3, whose functions and positions within the structure are unknown. The N-terminal glycine of the VP2 coat protein has been shown to be cotranslationally acylated with myristic acid. To study the function of this modification and the role of VP2 in the life cycle of polyomavirus, the N-terminal glycine, critical to the myristylation consensus sequence, has been altered to a glutamic acid or a valine residue by site-directed oligonucleotide mutagenesis. The glycine----glutamic acid mutant DNA has been further studied. When transfected into cells permissive for the polyomavirus full lytic life cycle, this mutant DNA replicated at levels comparable to those of wild-type viral DNA, and small amounts of nonrevertant (mutant) virus could be harvested from the cultures. The virus particles viewed by electron microscopy appeared slightly distorted, but the ratio of full to empty particles was similar to that produced in a wild-type viral infection. Mutant virus was capable of reinfecting permissive cells but with a considerably reduced efficiency.

Recombination resulting in unusual features in the polyomavirus genome isolated from a murine tumor cell line

Polyomavirus-induced tumor formation in the adult natural mouse host has been investigated. Tumors were produced in nude mice with the transformation-defective mutant strain NG18 after a long latency period by apparent activation of a cryptic endogenous transforming viral function. A tumor cell line, designated ScB, was established and characterized. Cells from this morphologically distinct line were unusual in that they grew in soft agar but did not form foci. They were highly tumorigenic. They had a 3.1-kilobase major viral transcript that hybridized to probes derived from regions encoding both the T antigens and the structural proteins. ScB cells expressed polyomavirus small T antigen, a slightly altered middle T antigen, and a truncated large T antigen but no capsid proteins. Middle T antigen preserved its interactions with host proteins of 60 and 37 kilodaltons and with c-src. Analysis of cDNA and genomic clones indicated that the stable viral insert in the ScB genome contained multiple copies of the viral B-enhancer. The genome contained two intragenic inversions which created novel early- to late-strand switches. A simple model for the generation of one inversion is proposed that involves the juxtaposition of two stem-loop structures at an illegitimate recombination site; the location of the inverted segment within the integrated sequence permits use of the viral late polyadenylation signal in early-region transcripts, as confirmed by DNA sequence. A repetitive sequence may facilitate recombination at the other inversion site. Both the biological consequences of the observed rearrangements and the structure of the integrated viral DNA suggest that the recombination events are nonrandom.

Immortalization and transformation of human fibroblasts by regulated expression of polyoma virus T antigens

We have established conditions for the immortalization of human fibroblasts by the large T antigen of the rodent virus polyoma. This allows the mechanism of immortalization to be studied, without interference by transformation events, in cells with relatively stable chromosomes. Large T antigen could immortalize human fibroblasts if expression was driven by a heterologous promoter like the immediate early promoter/enhancer of cytomegalovirus or the inducible mouse mammary tumour virus (MMTV) promoter. Using the latter promoter and dexamethasone, three clones were obtained, the immortalized phenotype of which was strictly dependent on the induction of T-antigen expression. At least one of these clones became mortal after removal of the inducing agent. The expression of large T antigen was paralleled by PCNA gene expression, as shown by nuclear run-off transcription, whereas none of a number of other known proto-oncogenes was influenced in its activity. Immortalized fibroblasts were readily transformed by polyoma virus middle T antigen expressed from the MMTV promoter or by the activated c-Ha-ras oncogene. The reversibility of immortalization and transformation is considered.

CpG methylation of viral DNA in EBV-associated tumours

In EBV-immortalized lymphoblastoid cell lines (LCLs) a small number of "latent" proteins are expressed. These are the EBV nuclear antigens, EBNAs 1-6, and a latent membrane protein, LMP. We have investigated the expression of these proteins in a variety of EBV-associated tumours and cell lines. Whereas transplant and B-cell lymphomas from cotton-top tamarins appear to express the full range of antigens found in LCLs, we and others have found that in Burkitt's lymphomas (BL) and a nasopharyngeal carcinoma (NPC) isolate, EBNA expression is restricted to EBNA-I. (In NPC, but not in BL, LMP may also be expressed). In order to ask what restricts the expression of EBNA 2-6 in NPC and BL cells it seemed reasonable to consider the possibility that the DNA sequences normally regulating expression of these antigens could be chemically modified. In this analysis, a tight inverse correlation between methylation of CpG dinucleotides in the 5' flanking region of the EBNA-2 gene and the expression of EBNAs 2-6 has been revealed. In the NPC tumour, CpG methylation within the gene is also observed, as is specific methylation over the EBNA-I region I and II binding sites (in oriP). The significance of these observations is considered.

Establishment of immortalized primate epithelial cells with sub-genomic EBV DNA

The genetic information in a sub-fragment of EBV DNA, designated p31 (containing less than a quarter of the viral genome and derived from a recombinant DNA cosmid library) allows epithelial cells from primary monkey and human kidney cultures to escape senescence under standard tissue culture conditions. A number of epithelial cell lines, designated M1/31, 483/31, 199/31 and HK/31, have been established and characterized following transfection of primary cells with p31 DNA. They share many properties, although morphologically they are not all identical. The cultures are immortalized but not fully transformed or tumorigenic. They appear to be phenotypically stable, although DNA hybridization studies indicate that genotypic alterations, including amplification, occur subsequent to transfection with p31 DNA and the establishment of a continuously proliferating epithelium. All cell lines consistently express high levels of cytokeratin 18 and varying amounts of cytokeratin 7, demonstrating their epithelial origin. From a single marmoset kidney (designated 199) a series of related immortalized cells, with subtle phenotypic differences, have been generated by p31 or sub-fragments of it. Although hallmarks of a "hit-and-run" mechanism are apparent in all of our studies, 2 different techniques (in situ hybridization or selection for cell survival in semi-solid media, followed by nucleic acid hybridization) show that, in late-passaged cultures, a small proportion of the cells still contain some viral DNA. The studies focus on genetic information within the BamHI A and I regions as being relevant to immortalization. The role of the EBV DNA fragment in the genesis of epithelial cell lines is considered.

Visualization of single copies of the Epstein-Barr virus genome by in situ hybridization

Conditions have been established for demonstrating small numbers of genes of the Epstein-Barr virus (EBV) in B-lymphoid cells by in situ hybridization using biotinylated EBV-specific DNA from cloned BamHI fragments of the viral genome. Single copies of EBV genomes were successfully visualized with minimal background when the probe concentration was 0.2 micrograms/ml, the DNA denaturation step was performed at 100 degrees C, and the immunochemical detection system employed a three-layer peroxidase protocol with gold-silver amplification of the diaminobenzidine substrate. The minimal target DNA detectable was about 10 kilobase pairs. In the case of sectioned cells fixed overnight with formalin, simulating conditions used in routine tissue fixation, this approach failed to demonstrate EBV DNA present at less than 100 copies per cell, that is, at the level found in Raji cells. However, when denaturation was performed using microwave irradiation with the other optimized conditions maintained, EBV DNA could be visualized in 10-20% of such cells, although not in cells known to contain fewer than 10 copies per cell. Thus, microwave irradiation partially overcomes the limit of DNA target detection imposed by formalin.

EBV gene expression in an NPC-related tumour

A nasopharyngeal carcinoma tumour (designated C15) propagated in nude mice has been used to generate a large cDNA library that we have analysed for Epstein-Barr virus (EBV) gene expression. No gross alterations exist in viral DNA from C15 relative to other human isolates and the large deletion present in the B95-8 'prototype' viral strain established in marmoset cells is not found; C15 contains no linear virion DNA. In the cDNA library, of the six EBV nuclear antigens (EBNAs) expressed in latently infected B-lymphocytes, only clones for EBNA-1 are found. These data are confirmed by immunoblotting. Sequence analysis shows the EBNA-1 mRNA splicing pattern in the carcinoma to differ from that observed in B-lymphocytes. Further, contrary to observations with B-cell lines, most viral transcription in the tumour is localized onto the 'rightmost' region of the conventional EBV physical map. Transcripts identified corresponding to known genes include those for the latent membrane protein (LMP), the alkaline DNA exonuclease and probably the terminal protein; major transcripts are also derived from the BamHI D fragment and the region deleted in B95-8 EBV DNA. Novel transcripts have also been identified that proceed in an anti-sense direction to genes encoding functions associated with replication, such as the viral DNA polymerase. They contain a large, hitherto unidentified, open reading frame in the viral genome that is complementary to the putative function known as BALF3 and a smaller open reading frame complementary to BALF5 (the DNA polymerase gene). From the present studies we can conclude that: (i) EBV transcription patterns in the epithelial cells vary markedly from those identified previously in B-cells, reflecting differential use of promoters or splicing patterns. (ii) Transcription is tightly regulated and restricted in the C15 tumour with many latent genes, notably EBNAs 2-6, being 'switched off.' (iii) A family of cytoplasmic RNAs are transcribed in an antisense direction to a number of existing open reading frames in the EBV genome. (iv) There are a number of mutations in C15 transcripts relative to the B95-8 genome, some of which could result in amino acid alterations in proteins.

Epstein-Barr virus latent gene expression during the initiation of B cell immortalization

Epstein-Barr virus (EBV) has the capacity to immortalize a subpopulation of resting B lymphocytes. Lymphoblastoid cell lines (LCL) established in this way carry the latent EBV genome as multiple copies of an extrachromosomal episome. Viral gene expression in LCLs is highly restricted; products identified correspond to a membrane protein (latent membrane protein; LMP), a nuclear antigen complex (Epstein-Barr nuclear antigens; EBNAs 1 to 6), two small RNA species (EBERs 1 and 2) and RNA species thought to encode a second membrane-associated polypeptide designated terminal protein (TP). Here we have investigated the temporal sequence of expression of the characterized 'latent' proteins during the initiation of immortalization when resting B cells are stimulated to enter and traverse the cell cycle. The analysis has been carried out on prolymphocytic leukaemia cells infected in vitro with either the immortalizing B95-8 strain of virus or the non-immortalizing P3HR1 strain. The results reveal that following B95-8 infection, a sequence of EBV expression is initiated within approximately 8 h with the synthesis of detectable levels of EBNA 2 shortly followed by EBNAs 1, 3, 4, 5 and 6. There is then a delay of approximately 40 h until the expression of LMP completes the latent pattern of proteins found in LCLs. P3HR1 infection, however, produces only transient expression of some EBNA species in a small percentage of cells after approximately 48 h. These observations suggest the failure of P3HR1 virus to immortalize may not be due solely to the absence of EBNA 2 expression and that cellular and/or virus-mediated events occur after EBNA synthesis which then facilitate efficient LMP expression and immortalization.

Persistence of Epstein-Barr virus in salivary gland biopsies from healthy individuals and patients with Sjögren's syndrome

Salivary gland biopsies from 12 patients with primary Sjögren's syndrome and 10 controls were examined for Epstein-Barr virus (EBV) DNA by in-situ hybridization and for EBV proteins by immunofluorescence and peroxidase techniques. Viral DNA was found in biopsies from two out of 12 patients with primary Sjögren's syndrome and six out of the 10 controls. The DNA and early antigen were in epithelial cells lining the ducts and acini, early antigen expression being limited to the luminal side of the epithelium. In eight biopsies studied with other antibodies, membrane antigen was identified in both acini and ducts but viral capsid antigen and Epstein-Barr nuclear antigen were not detected. EBV was found in biopsies from five of the controls without inflammation or Class II expression. This suggests that, in health, persistence and replication occur without inducing an immune response, possibly due to the restricted expression of early antigen on the luminal of the epithelium, away from immune surveillance. The inflammation in Sjögren's syndrome could be due to a breakdown of this unusual mechanism for viral persistence leading to a vigorous immune response to the virus. However our study provides no evidence to suggest that EBV infection load is increased in this disease.

Expression of biologically active middle T antigen of polyoma virus from recombinant baculoviruses

Two different recombinant baculoviruses have been generated for expressing the middle T antigen (MT) of polyoma virus in insect (Sf9) cells. One (pAcI-PyMT) produces moderate levels of MT and the other (pVL-PyMT) high levels. Indirect immunofluorescence and cellular fractionation studies with pAcI-PyMT infected Sf9 cells give results similar to those observed with wild type polyoma virus infected mouse cells, and show MT to be mainly associated with cytoplasmic membranes in the insect cell. In the latter, a sub-population of MT is phosphorylated in in vitro protein kinase assays. The yields of MT from pVL-PyMT infected cells are high enough to suggest that this protein can now be produced by this method in sufficient amounts for definitive biochemical and crystallographic analyses.

Prediction and demonstration of a novel Epstein-Barr virus nuclear antigen

The protein sequence predicted by the Epstein Barr virus (EBV) BERF4 open reading frame includes a tetrapeptide, Lys-Arg-Pro-Arg (KRPR), shown for other proteins to be a component of a signal for rapid nuclear localization. A subgenomic fragment of EBV DNA containing BERF4 has been incorporated into an expression vector, transfected onto primate cells and the nuclear distribution of the resulting protein established by immunofluorescence using EBV positive human sera. These sera contained high titres of antibodies to a fusion protein, produced in E. coli, consisting of beta-galactosidase and the C-terminal 167 amino acids of BERF4. Immunoaffinity purified antibodies reactive with the EBV component of the fusion show the molecular weight of this antigen in EBV immortalized B-cell lines to be about 160 kD. The demonstration that BERF4 contains an exon encoding a nuclear protein identifies a new EBNA gene (EBNA-6) and suggests that KRPR is a signal sequence common to a number of viral and cellular nuclear polypeptides which bind to nucleic acids and may therefore be of predictive value in identifying karyophilic proteins.