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Rak, a novel nuclear tyrosine kinase expressed in epithelial cells
W G Cance1, R J Craven, M Bergman
1Department of Surgery, University of North Carolina at Chapel Hill School of Medicine 27599.
Abstract:
We have identified a new tyrosine kinase from human breast cancer cells called Rak (a Russian word for cancer) that shares 51% identity with c-Src. Sequencing of the full-length complementary DNA revealed that Rak is a tyrosine kinase with a molecular weight of 54,000 that contains SH2 and SH3 domains, as well as tyrosine residues analogous to the autophosphorylation and regulatory tyrosines of the Src family. Biochemical and site-directed mutagenesis analyses revealed that a carboxy-terminal peptide of p54rak was phosphorylated by a cytoplasmic tyrosine kinase (CSK) and that, as in the Src family, it is the COOH-terminal tyrosine that is phosphorylated by CSK. However, there were some properties of Rak that are distinct from Src-like kinases: (a) expression of Rak was predominantly in epithelial-derived cell lines and tissues, especially normal liver and kidney, and cell lines of breast and colon origin; (b) Rak does not harbor the NH2-terminal glycine essential for myristylation and membrane localization; and (c) Rak possesses a putative bipartite nuclear localization signal in the SH2 domain, and subcellular fractionation studies revealed that p54rak resides predominantly in the nucleus. In addition, p54rak was overexpressed in subsets of primary human epithelial tumors, suggesting that p54rak may have a role in human cancer. Thus, Rak is a novel epithelial-associated nuclear tyrosine kinase that may represent a unique subfamily of the Src-related kinases.
Insights
Researchers discovered Rak, a novel tyrosine kinase in human breast cancer cells. This epithelial-associated nuclear kinase may play a role in cancer development.
Area of Science:
- Molecular Biology
- Oncology
- Biochemistry
Background:
- The Src family of tyrosine kinases are crucial regulators of cell signaling.
- Dysregulation of tyrosine kinases is implicated in various cancers.
- Understanding novel kinases like Rak is essential for cancer research.
Purpose of the Study:
- To identify and characterize a novel tyrosine kinase, Rak, from human breast cancer cells.
- To investigate the structural, biochemical, and subcellular localization properties of Rak.
- To explore the potential role of Rak in human epithelial cancers.
Main Methods:
- Complementary DNA sequencing to determine Rak's full-length sequence.
- Biochemical assays and site-directed mutagenesis to analyze Rak's phosphorylation.
- Subcellular fractionation to determine Rak's cellular localization.
- Analysis of Rak expression in normal tissues and cancer cell lines.
Main Results:
- Rak is a 54,000 molecular weight tyrosine kinase sharing 51% identity with c-Src, possessing SH2 and SH3 domains.
- Rak is phosphorylated by cytoplasmic tyrosine kinase (CSK) at its COOH-terminal tyrosine.
- Rak is predominantly expressed in epithelial cells, lacks myristylation signals, and localizes to the nucleus.
- p54rak is overexpressed in subsets of primary human epithelial tumors.
Conclusions:
- Rak is a novel epithelial-associated nuclear tyrosine kinase, potentially representing a unique subfamily of Src-related kinases.
- Rak's distinct properties, including nuclear localization and epithelial cell specificity, differentiate it from Src-like kinases.
- Overexpression of Rak in epithelial tumors suggests its involvement in human cancer pathogenesis.