PubMed Health⌕ Search

Biomedical subjects

K Ballmer-Hofer

Publications and source records attributed to K Ballmer-Hofer.

42 records · Page 3Linked to original sources
PubMed1988

Association of polyoma virus middle T antigen and pp60src with cytoskeletal elements.

Polyoma virus middle T antigen (mT) as well as pp60v-src, the oncogene product of Rous sarcoma virus, associate with membranes and cytoskeletal structures in virus-infected cells. Furthermore, mT has been shown to be tightly bound to pp60c-src, the cellular homolog of pp60v-src. While the presence of pp60v-src in focal contacts has been demonstrated, the localization of mT and pp60c-src is less clear. We show here that mT is associated with focal contacts and that the phosphorylated protein can be immunoprecipitated with anti-vinculin serum. We suggest that the mT/pp60c-src complex and pp60v-src are tightly bound to vinculin. This interaction may be relevant in the process of cell transformation since disorganization of the actin filament network seems to play an important role in the deregulation of cell growth observed in tumor cells.

Animals↗
PubMed1987

Expression of influenza hemagglutinin-polyoma T-antigen fusion proteins in a rat embryo fibroblast cell line.

Plasmids encoding the amino terminal portion of an influenza virus hemagglutinin (HA) fused to polyoma virus middle T (mT) or large T (lT) sequences have been constructed. Stable expression of the chimeric proteins was obtained in established rat embryo fibroblasts following plasmid co-transfection and selection for G418 resistance. The synthesis and localization of the proteins was followed by metabolic labeling with [35S]methionine and [3H]mannose, cell fractionation, and immunoprecipitation with anti-polyoma T antibody. The HA leader and amino terminal peptide direct the synthesis of the lT and mT proteins into the endoplasmic reticulum where they undergo glycosylation, but this occurs with a very low efficiency. Most of the HA-mT and HA-lT fusion protein molecules do not enter completely into the endoplasmic reticulum, but rather achieve their normal locations in the cell as slightly higher molecular weight proteins, presumably due to the extra sequences derived from HA at their amino termini. HA-mT fusion protein is found to have associated tyrosine-specific protein kinase activity precipitable with anti-src as well as anti-T antibody, and cells expressing this fusion protein have a transformed phenotype.

Animals↗
PubMed1987

Variations in polyoma virus genotype in relation to tumor induction in mice. Characterization of wild type strains with widely differing tumor profiles.

The authors have explored the effects of variations in mouse polyoma virus genotype on patterns of tumor formation in the mouse. Four "wild type" virus strains were surveyed. Two were highly oncogenic, inducing multiple tumors of epithelial and mesenchymal origin, at high frequency and with short latency. The other two strains were weakly oncogenic, inducing fewer tumors, solely of mesenchymal origin, and after a long latency. These sharply contrasting tumor profiles were reproduced with virus stocks derived from molecularly cloned viral genomes. Though vastly different in their oncogenic properties, these cloned viruses proved equally effective in transforming established rat fibroblasts in culture and showed the same patterns of tumor antigen expression in cultured mouse cells. Complexes of polyoma middle T antigen and pp60c-src were demonstrated in extracts of epithelial tumors induced by a highly oncogenic virus strain. It is concluded that polyoma viral genetic determinants for tumor induction in the mouse are more complex than those previously defined by the use of cell transformation systems.

Animals↗
PubMed1985

Phosphorylation of polyoma middle T antigen and cellular proteins in purified plasma membranes of polyoma virus-infected cells.

We have studied phosphorylation carried out by purified plasma membranes from polyoma virus-infected cells. When isolated membranes are incubated with [gamma-32P]ATP, polyoma virus middle T antigen (mT) becomes phosphorylated on tyrosine. Partial proteolysis mapping shows the same pattern as previously noted for mT labeled in immune complexes. Membranes labeled in vitro were also extracted and immunoprecipitated with anti-T or anti-src antibody. With either antibody, both mT and pp60c-src were brought down and shown to be labeled on tyrosine. The mT of an hr-t mutant (NG59) showed only a trace amount of labeling in membranes under the same conditions. Proteins from infected and uninfected cell membranes labeled in vitro were separated on two-dimensional gels. An acidic 40-kd phosphoprotein was labeled in uninfected cell membranes, but was not seen using membranes from wild-type virus-infected cells. Neither NG59, which encodes a defective but membrane-associated mT, nor a mutant encoding a truncated mT that fails to associate with membranes, alters the level of the 40-kd phosphoprotein in membranes labeled in vitro. These results suggest that mT, acting through pp60c-src and possibly other cellular kinases and phosphatases, can affect cell protein phosphorylation as part of the transformation process.

Adenosine Triphosphate↗
PubMed1985

Virion assembly defect of polyomavirus hr-t mutants: underphosphorylation of major capsid protein VP1 before viral DNA encapsidation.

The major capsid protein of polyomavirus, VP1, was separated into at least four subspecies by isoelectric focusing. One of these subspecies was selectively extracted from purified virions by mild treatment with sodium dodecyl sulfate, leaving a 140S particle enriched in the other three forms. The two most acidic subspecies were labeled in vivo with [32P]phosphate, and these subspecies are among those identified as being deficient in nontransforming host range (hr-t) mutant virus nonpermissive infection of NIH3T3 cells. Quantitation of VP1 phosphorylation revealed that hr-t mutant virus VP1 is phosphorylated to about 40 to 50% the level of the wild type in NIH3T3 cells, and two-dimensional phosphoamino acid analysis suggested that threonine phosphorylation was affected more than serine phosphorylation. Two results indicate that the VP1 modifications occur before and independent of virus assembly: modified subspecies were detected during wild-type infection within a 2-min pulse-label with [32S]methionine, and VP1 modifications of temperature-sensitive VP1 mutants were the same at both restrictive and permissive temperatures for virus assembly. We conclude that most VP1 modification occurs before viral DNA encapsidation, and that one defect in hr-t mutant virus assembly is in VP1 phosphorylation, primarily affecting threonine.

DNA, Viral↗