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.

Background: Postsynaptic density (PSD)-95 interacts with and mediates clustering of the N-methyl-daspartate-receptors (NMDA-R). PSD-95 also interacts with the hDLG-associated protein DAP, which is also called Synapse-associated protein 90-associated protein (SAPAP), and Guanylate kinase-associated protein (GKAP).

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We report here that Streptomyces coelicolor A3(2) produces at least seven butyrolactone autoregulators: two of the IM‐2 type, four virginiae butanolide type, and one A‐factor type. The most abundant one corresponds to virginiae butanolide‐C9 having a C2 side chain of nine carbons. Model butyrolactone compounds as well as extracts of S. coelicolor mycelia showed clear induction of morphological differentiation, implying that S. coelicolor A3(2) probably possessed butyrolactone‐type autoregulator(s) controlling the morphological differentiation.