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A comparison of topoisomerase I activity in normal and transformed cells
Abstract:
Many viral oncogenes encode protein-tyrosine kinase activities. However, important in vivo substrates of these enzymes have yet to be identified. Recently, type I topoisomerases were shown to be in vitro substrates for two tyrosine kinases. Following tyrosine phosphorylation, topoisomerase I activity was reduced 10-fold (Tse-Dinh et al. Nature 312: 785-786, 1984). To determine whether topoisomerase I activity was modulated by tyrosine phosphorylation in vivo, we have measured topoisomerase I activity in nuclear lysates prepared from both normal fibroblasts and cells transformed by two different viral oncogenes (v-abl, v-src). Under a variety of experimental conditions, we have found no evidence to support the notion that type I topoisomerase activity is modulated by tyrosine phosphorylation in vivo.
Insights
This study investigated if tyrosine phosphorylation affects topoisomerase I activity in cells. Researchers found no evidence that this enzyme
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Viral oncogenes often encode protein-tyrosine kinases.
- In vivo substrates for these kinases remain largely unidentified.
- Type I topoisomerases were previously shown as in vitro substrates, with phosphorylation reducing activity 10-fold.
Purpose of the Study:
- To determine if type I topoisomerase activity is modulated by tyrosine phosphorylation in vivo.
- To investigate the role of viral oncogenes in this potential modulation.
Main Methods:
- Preparation of nuclear lysates from normal fibroblasts and cells transformed by v-abl and v-src oncogenes.
- Measurement of type I topoisomerase activity in these lysates.
- Analysis under various experimental conditions.
Main Results:
- No evidence was found to support that type I topoisomerase activity is modulated by tyrosine phosphorylation in vivo.
- The in vitro findings regarding phosphorylation-induced inhibition were not replicated in vivo.
Conclusions:
- Tyrosine phosphorylation does not appear to regulate type I topoisomerase activity in the tested cellular models.
- Further research may be needed to identify other in vivo substrates of viral tyrosine kinases.