Oligomerization of simian virus 40 large T antigen is not necessarily repressed by temperature-sensitive A gene lesions

Simian virus 40 large T antigen is a multifunctional protein which exists in different molecular weight forms. According to several reports, T antigen encoded by temperature-sensitive simian virus 40 A locus mutants (tsA) is unable to oligomerize into high-molecular-weight species. To try to correlate structural and functional properties, we selected tsA58 and tsA1499, both of which are heat sensitive for lytic growth, but only tsA58 is heat sensitive for transformation. Here we report that at permissive and nonpermissive temperatures, T antigen from tsA1499-infected monkey cells retained the ability to oligomerize, whereas reported previously, tsA58 T antigen failed to oligomerize at the nonpermissive temperature. Furthermore, we studied the formation of complexes between T antigen and the cellular p53 protein (T-p53) late in infection. Corresponding to its heat-stable oligomerization properties, T antigen encoded by tsA1499 formed T-p53 complexes regardless of temperature. In contrast, tsA58 encoded T-p53 complexes, preformed at the permissive temperature, remained heat stable after shifting up to the nonpermissive temperature; but at this temperature no new T-p53 complexes arose. The mutants did not replicate viral DNA at the nonpermissive temperature, suggesting that neither the oligomerization of T antigen nor the formation of T-p53 complexes seems to be sufficient for viral DNA replication or for the expression of late viral proteins.

Tightly associated lipids may anchor SV40 large T antigen in plasma membrane

Simian virus 40 (SV40) large T antigen, a multifunctional protein necessary for lytic growth and cell transformation, is located mainly in the nucleus and in small amounts on the cell surface (surface T). Surface T may have a passive role in SV40 tumour rejection by cytotoxic T cells as a component of SV40-TSTA (tumour-specific transplantation antigen). The unusual induction of this immune response by immunizing mice with soluble T antigen led us to investigate the in vitro binding of T antigen to the surface of living cells in more detail. Our results show that native surface T and a minor subset of large T antigen having a high cell surface binding affinity in vitro, behave like integral membrane proteins. Several viral proteins including SV40 T antigen and cellular proteins seem to be linked to fatty acids (acylation). To analyse whether this mechanism is involved in the stable attachment of in vitro-bound T antigen to the plasma membrane of living target cells, we determined the degree of labelling of this molecule by using target cells prelabelled with 3H-fatty acid. Here we report that T antigen extracted from unlabelled SV40-transformed cells (SV80) becomes 3H-labelled after in vitro binding to the cell surface of 3H-palmitate-prelabelled HeLa cells. These results suggest that T antigen attached externally to living cells, may be anchored by tightly linked lipids.

A subclass of simian virus 40 T antigen with a high cell surface binding affinity

SV40 virus-infected and -transformed cells express large T antigen on the cell surface (surface T). In the present study the cell surface binding properties of T antigen extracted from SV40-transformed cells were investigated. Only small amounts of T antigen with tight cell surface binding properties were efficiently removable by absorption on living cells from the majority of T antigen detectable in cell extracts. As shown in immunofluorescence microscopy both native surface T and experimentally in vitro cell surface bound T antigen were stained in similar microcluster patterns. Comparative SDS-polyacrylamide gel electrophoretic analysis indicated that T antigen extracted from SV40-transformed cells and in vitro cell surface bound T antigen had the same apparent molecular weight of approximately 90,000 da. A quantitative 125I-protein A binding assay using antisera directed against purified T antigen demonstrated that a metal-ion chelating agent (EDTA) or hypertonic salt solutions were unable to remove surface T or in vitro cell surface bound T antigen from living cells. In contrast, both antigens could be solubilized by detergents. Moreover, both types of cell surface associated T antigens seemed to be metabolically stable. Altogether, one can postulate a minor subclass of T antigen with a tight binding affinity to the cell surface of living cells. According to these properties this experimentally membrane bound subclass, as well as native surface T, seem to belong to the class of integral rather than peripheral membrane proteins.

Self-assembly of simian virus 40 large T antigen oligomers by divalent cations

In simian virus 40-transformed cells, simian virus 40 large T antigen can be detected in different forms separable by sucrose density gradient centrifugation. In our experiments, light forms sedimented around 5 to 7S, oligomers such as tetramers were detected around 16S, and higher aggregates sedimented in a broad distribution reaching above 23S. The oligomers sedimenting at and above 16S could be disassembled into the slowly sedimenting 5 to 7S forms by chelating agents [EDTA or ethylene bis(oxonitrilo)tetraacetate]. After the addition of divalent cations (CaCl2 or MgCl2) in excess of chelating agents, oligomeric forms reassembled and appeared in a sedimentation pattern resembling that observed before treatment with chelating agents. Time course studies permitted the identification of the 5 to 7S forms as precursors upon pulse-labeling (15 min); the 16S and higher oligomers were identified as the successors after a 14-h chase. Treatment of extracts of pulse-chase-labeled cells with chelating agents again disassembled the oligomers, whereas pulse-labeled precursors did not change their 5 to 7S sedimentation pattern. Adding an excess of divalent cations reassembled the pulse-chase-labeled T antigen to oligomers but did not influence the sedimentation behavior of pulse-labeled 5 to 7S precursors. It is therefore reasonable to assume that a posttranslational modulation induces divalent cation binding, leading finally to the oligomerization of T antigen. Thus, some of the multifunctional activities of T antigen can be dictated by divalent cation binding properties.

Disaggregation and reconstitution of oligomeric complexes of simian virus 40 large T-antigen

Biochemical properties of multiple species of simian virus 40 (SV40) large T-antigen produced in SV40-infected monkey cells were investigated by zonal sedimentation centrifugation of radiolabelled cell extracts on sucrose gradients. Two major subpopulations of T-antigen detected by immunoprecipitation and gel electrophoresis could be distinctly separated: low molecular weight forms ranging approximately between 5S and 10S and higher oligomeric forms at about 16S to 23S. Removal of divalent cations by chelating agents such as EDTA disassembled higher oligomers into low molecular weight forms (5S to 10S). Adding divalent cations in excess of the EDTA concentration reassembled the higher oligomeric forms, showing a sedimentation behaviour like T-antigen in untreated cell extracts. Our results are compatible with the hypothesis that divalent cation binding properties of T-antigen participate in the natural pathway of assembling multiple oligomeric species.

The binding of simian virus 40 large T antigen to the polyphosphate backbone of nucleic acids

Simian virus 40 (SV40) large tumor antigen (T antigen), a phosphoprotein found in nuclei of SV40-infected and -transformed cells, binds nonspecifically to DNA. To study this mechanism the binding properties of T antigen to double-stranded (ds) and single-stranded (ss) DNA-cellulose as well as to phosphocellulose were compared. After incubation of [35S] methionine or [3H] leucine/[32 P] phosphate radioactively-labeled cell extracts at different pH values (6.0, 7.3, 9.0) with DNA- or phosphocellulose, bound and unbound species of T antigen were purified and analyzed by SDS-polyacrylamide gel electrophoresis for both the yield and the possible correlation with protein phosphorylation. T antigens bound with comparable affinities to ds- and ss-DNA-cellulose and phosphocellulose. These results suggest the binding of T antigen to the polyphosphate backbone of DNA as a molecular mechanism for its nonspecific binding. The evidence for this observation was supported by blocking the binding of T antigen to DNA-cellulose by divalent cations (Ca2+, Mg2+). 3H/32P ratios of T antigen obtained by double-labeling cells for various times imply that higher phosphorylated forms of T antigen bound more strongly to ds- and ss-DNA as well as to phosphocellulose. Thus, in the presence of cellular proteins and other components the binding activity of T antigen to the polyphosphate backbone of DNA seems to be positively correlated with its phosphorylation. These observations are consistent with the hypothesis that the binding affinities of SV40 T antigen to host cell DNA may be regulated by its phosphorylation.

Detection of simian virus 40 T-antigen-related antigens by a 125I-protein A-binding assay and by immunofluorescence microscopy on the surface of SV40-transformed monolayer cells

Simian virus 40 (SV40)-transformed cells express the SV40-specific tumour transplantation antigen (TSTA) on the cell surface and the SV40-coded tumour antigen in their nuclei. TSTA is defined by SV40-specific transplantation immunity, whereas T-antigen (T-Ag) can be detected serologically by indirect immunofluorescence. Both antigens, however, are derived from the A gene of SV40. We therefore analysed SV40-transformed cells for the presence of serologically detectable T-Ag-related molecules. Such antigens could not be detected on the surface of living SV40-transformed cells in monolayers. However, after a short formaldehyde fixation it was possible to stain the cell surfaces of SV40-transformed cells with sera from rabbits immunized with purified SDS-denatured T-Ag, but not with sera from hamsters bearing SV40-induced tumours. T-Ag-related antigens could be detected with both types of antisera by applying a more sensitive 125I-protein A assay. The T-Ag specificity of the binding of hamster SV40 tumour sera was demonstrated be a 125I-IgG-blocking assay in which preincubation of formaldehyde-fixed SV40-transformed cells with rabbiet anti-SDS-T-Ag serum inhibited the binding of hamster SV40 tumour serum by about 70%. The localization of T-Ag-related antigens on the outside of plasma membranes of formaldehyde-fixed cells was shown by an anti-SDS-T-Ag serum-specific binding of fluorescein isothiocyanate-labelled Staphylococcus aureus to the cell surface. Out results are consistent with the hypothesis that SV40 T-Ag-related antigens are involved in the formation of TSTA.

Timolol and hydrochlorothiazide-amiloride in primary hypertension

Fifty-five patients with primary hypertension, World Health Organization (WHO) stages I and II, were randomly allocated to a 9-mo multicenter, controlled, double-blind, crossover study with timolol, a nonselective beta adrenoceptor blocker, and hydrochlorothiazide combined with the potassium-sparing drug amiloride (AHCT). In 54% of the patients the blood pressure responded to timolol, in 87% to AHCT, and in 91% to a combination of the two. The diurectic was more effective than the beta blocker in patients with low-renin hypertension, who all responded to AHCT. Overall, there was no correlation between pretreatment plasma renin activity (PRA) and hypothensive effect of either drug. Timolol reduced PRA by 58% and plasma aldosterone (PA) by 23% while AHCT increased these levels threefold. Combination therapy increased PA while PRA returned towards baseline, suggesting greater aldosterone stimulation by the diuretic component. Serum triglycerides rose during timolol treatment alone and in combination. Both timolol and AHCT are effective antihypertensives. In combination they normalize blood pressure in most patients with primary hypertension (WHO stages I and II). Determination of PRA is useful as a guide to the choice of the first treatment in searching out low-renin hypertensive patients, who are best treated with diuretics.

Simian virus 40 T-antigen-related cell surface antigen: serological demonstration on simian virus 40-transformed monolayer cells in situ

Simian virus 40 (SV40)-transformed monolayer cells were analyzed in situ by indirect immunofluorescence microscopy for the postulated cell surface location of SV40 T-antigen-related molecules. With antisera prepared against purified, sodium dodecyl sulfate-denatured SV40 T-antigen, positive surface staining was obtained when the cells had been treated with formaldehyde before immunofluorescence analysis. In contrast, living SV40-transformed cells analyzed in monolayer were surface fluorescence negative. The fixation procedure developed in this study combined with a double staining immunofluorescence technique allowed the simultaneous analysis of the same cells for the expression of both SV40 T-antigen-related surface antigen and nuclear T-antigen. The localization of SV40 T-antigen-related surface antigen on the outer surface of the plasma membrane of formaldehyde-fixed SV40-transformed cells was demonstrated directly by the protein A-mediated binding of Staphylococcus aureus bacteria on formaldehyde-fixed SV40-transformed cells precoated with antiserum against sodium dodecyl sulfate-denatured T-antigen. Both cell surface staining and S. aureus binding were found to be highly specific for SV40 T-antigen-related binding sites. These results indicate that T-antigen-related molecules in a cryptic form are located on the surface of SV40-transformed monolayer cells and can be detected in situ after modification of the cell surface architecture.