br Acknowledgements br Introduction Protein tyrosine phospho

Acknowledgements
Introduction Protein–tyrosine phosphorylation, catalyzed by protein–tyrosine kinases and protein–tyrosine phosphatases, has a pivotal role in the regulation of a wide variety of cellular processes. Similar numbers of tyrosine kinases and tyrosine phosphatases are encoded by the human genome, assuming that they have comparable substrate specificities [1]. Tyrosine phosphorylation is a primarily cytoplasmic event [2]. In contrast to receptor-type tyrosine kinases, the functions of non-receptor-type tyrosine kinases deeply depend upon their subcellular localizations [3]. The Src-family of non-receptor-type tyrosine kinases consists of proto-oncogene products and related proteins, and is involved in a diverse array of signal transduction events [4]. Src kinases are found to localize to various subcellular compartments in addition to the plasma membrane. For instance, c-Src localizes to perinuclear membranes [5], and endosome membranes and the microtubule-organizing center [6]. Fyn associates with the centrosomes, the mitotic spindles and poles [7]. Lyn is present in the Golgi caveolin [8], chromosomes, spindles and the nuclear matrix [9]. The tyrosine kinase activity of Src-family kinases is tightly regulated by tyrosine phosphorylation and dephosphorylation [10]. Upon tyrosine phosphorylation at the C-terminal tail of Src kinases, the catalytic activity is repressed by the intramolecular interaction between the C-terminal phosphotyrosine residue and the SH2 domain [3]. The C-terminal tyrosine phosphorylation of Src kinases is catalyzed by Csk-family kinases, Csk [11] and its homolog Chk (Csk homologous kinase) [3]. Csk-family kinases are composed of the Src homology (SH) 3 domain, the SH2 domain and the tyrosine kinase domain, but lack the negative C-terminal tyrosine residue, autophosphorylation and myristoylation sites found in Src kinases. Csk and Chk partly exhibit distinctive features. Chk selectively suppresses the kinase activity of Lyn but not c-Src in platelets and megakaryocytic Dami 8-CPT-Cyclic AMP sodium salt australia [12], [13], although Csk negatively regulates the kinase activity of all members of Src kinases [14]. Upon negative regulation of Src kinases at the plasma membrane, Csk is recruited to the plasma membrane via its binding to Csk binding protein (Cbp) that has a transmembrane region [15], whereas Chk binds to growth factor receptors to be recruited to the plasma membrane [16]. Given that Chk-induced inhibition of cell proliferation via its association with ErbB-2 receptor and suppression of Src activity has been reported in breast cancer cells [17], [18], inhibition of Src kinases by Chk at the plasma membrane is generally believed to contribute to Chk-induced inhibition of cell proliferation. However, we recently showed that expression of Chk, which is distributed to the nucleus and the cytoplasm, inhibits proliferation of COS-1 cells [9], [19].
Materials and methods
Results
Discussion Despite the lack of amino-terminal acylation, Csk can relocate from the cytoplasm to the plasma membrane through its association with the transmembrane phosphoprotein Cbp that is tyrosine-phosphorylated by Src-family kinases [15]. By this association, Csk phosphorylates the C-terminal tyrosine residue of Src-family kinases at the plasma membrane [26]. Like Csk, Chk lacks acylation sites such as myristoylation and palmitoylation sites. In contrast, Chk binds to growth factor receptors, including c-Kit, ErbB-2 and TrkA, through its SH2 domain for relocation from the cytoplasm to the plasma membrane [27], [28], [29], [30]. Chk but not Csk is appreciably found in the nucleus (Fig. 1A), suggesting that Chk may possess an element/structure for its nuclear localization, despite the lack of a typical NLS. Our results demonstrate the significance of the N-terminal unique domain of Chk for its association with the nucleus including the nuclear matrix (Fig. 1, [2], Fig. 2, [3], Fig. 3). We further showed that the second half (N25–47) of the N-terminal unique domain plays an important role in the association with the nucleus. Given that Csk, unlike Chk, lacks an N-terminal extension domain, the difference in the functions between Csk and Chk may be partly attributed to the presence of the N-terminal domain of Chk.