Association of Lyn Tyrosine Kinase with the Nuclear Matrix and Cell‐Cycle‐Dependent Changes in Matrix‐Associated Tyrosine Kinase Activity

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Abstract
The nuclear matrix isolated from HeLa cells and Rat2 fibroblasts harbor? tyrosine kinase and tyrosine phosphatase activities. Polypeptides of 53, 56 and 60 kDa, associated with this subnuclear structure, were phosphorylated at tyrosine in vivo. By immunoblot and immunolabelling experiments, we identified one of the nuclear‐matrix‐associated tyrosine kinases as Lyn, a Src family member. Lyn was distributed as foci throughout the matrix. The p56 and p53 isoforms of Lyn remained firmly associated with the nuclear matrix after a variety of matrix preparation procedures, and were not detectable in the chromatin fraction of the nucleus. The tyrosine kinase activity associated with the nuclear matrix showed cell‐cycle‐dependent changes, maximum activity being observed at the G1/S transition phase. Polyoma‐virus‐transformed rat fibroblast cells showed sixfold higher tyrosine kinase activity in the nuclear matrix preparations compared to that in untransformed cells. These observations are consistent with the suggestion that tyrosine kinase activity associated with the nuclear matrix may be an important determinant of cellular proliferation.