Transmembrane phosphoprotein Cbp regulates the activities of Src-family tyrosine kinases

Abstract

The Src family of protein tyrosine kinases (Src-PTKs) is important in the regulation of growth and differentiation of eukaryotic cells. The activity of Src-PTKs in cells of different types is negatively controlled by Csk, which specifically phosphorylates a conserved regulatory tyrosine residue at the carboxy-terminal tail of the Src-PTKs. Csk is mainly cytoplasmic and Src-PTKs are predominantly membrane-associated. This raises a question about the mechanism of interaction between these enzymes. Here we present Cbp--a transmembrane phosphoprotein that is ubiquitously expressed and binds specifically to the SH2 domain of Csk. Cbp is involved in the membrane localization of Csk and in the Csk-mediated inhibition of c-Src. In the plasma membrane Cbp is exclusively localized in the GM1 ganglioside-enriched detergent-insoluble membrane domain, which is important in receptor-mediated signalling. These findings reveal Cbp as a new component of the regulatory mechanism controlling the activity of membrane-associated Src-PTKs.

Src family tyrosine kinases-driven colon cancer cell invasion is induced by Csk membrane delocalization

“…For example, the cytoskeletal-associated protein, paxillin, recruits Csk to the focal adhesion sites for the regulation of cell migration (Schaller and Parsons, 1995), whereas the signalling protein, Dok-1, induces Csk translocation to the plasma membrane for mitogenic regulation (Zhao et al, 2006). Additional Csk-binders have been identified, including the structural protein of caveolae, caveolin-1 (Lee et al, 2000), the junctional proteins VE-cadherin (Baumeister et al, 2005) and ZO-1 and the transmembrane protein phosphoprotein associated with glycosphingolipidenriched microdomain (PAG)/Csk-binding protein (Cbp) (Brdicka et al, 2000;Kawabuchi et al, 2000) localized in cholesterol-enriched membrane domains or rafts (named PAG in this study). Interestingly, SFK phosphorylation of these proteins triggers their binding to Csk through a SH2-pTyr-dependent mechanism, which creates a negative feedback regulatory loop.…”

Altered lipid raft–associated proximal signaling and translocation of CD45 tyrosine phosphatase in B lymphocytes from patients with systemic lupus erythematosus

“…A current model for the role of Lyn infers that COOH-terminal Src kinase (Csk) negatively regulates Lyn by phosphorylating the COOH-terminal negative regulatory tyrosine at position 507 (Y-507) (19). In contrast, a membrane protein tyrosine phosphatase, CD45, positively regulates Lyn by dephosphorylating Y-507, which causes a conformational change that reveals the catalytic domain of Lyn and promotes autophosphorylation of the positive regulatory tyrosine at position 396 (Y-396) (20,21).…”

Csk-binding protein (Cbp) negatively regulates epidermal growth factor-induced cell transformation by controlling Src activation

“…Expression of CbpY317F and Cbp knockdown diminishes the amount of Csk within microdomains Cbp, EGFR and Src have been reported to be localized within the microdomains (Lisanti et al, 1994;Smart et al, 1995;Kawabuchi et al, 2000). As shown in Figure 4a, Cbp, EGFR, and Src were all enriched in the same fractions where the microdomain-characteristic protein caveolin-1 was present.…”

“…It has been reported that a point mutation of murine Cbp at the Tyr314 residue abrogated its binding to Csk (Kawabuchi et al, 2000). A construct of human Cbp was generated in which Tyr317, the counterpart of Tyr314 in murine Cbp, was substituted with phenylalanine (CbpY317F).…”

“…Csk (C-terminal Src kinase)-binding protein (Cbp), which is also called phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG), is a ubiquitously expressed transmembrane adaptor protein (Kawabuchi et al, 2000;Brdicka et al, 2000). Cbp is exclusively localized to microdomains, where Src family kinase (SFK) is also found to be enriched (Simons and Ikonen, 1997;Brdicka et al, 2000).…”