Structural and Functional Studies of the Density Enhanced Receptor-like Protein Tyrosine Phosphatase DEP-1

Sammanfattning: Tyrosine phosphorylation is a central mechanism in cellular signalling leading to proliferation, migration or differentiation. Protein tyrosine phosphorylation is regulated by the coordinated actions of protein tyrosine kinases and protein tyrosine phosphatases. This thesis investigates the involvement of tyrosine phosphatases in contact-induced growth inhibition of cells. Furthermore, it describes the structure and function of the extracellular domain of the receptor-like tyrosine phosphatase DEP- 1. Tyrosine phosphatases, negatively regulating tyrosine kinases, have been suggested being involved in contact-induced growth inhibition of cells. Both endogenous EGF-receptors and transfected PDGF receptors showed a decreased ligand-induced tyrosine phosphorylation in cells of dense cultures. This difference was found to be due to increased receptor-directed tyrosine phosphatase activity in dense cultures. Density enhanced tyrosine phosphatase-1 (DEP-1) is a receptor-like tyrosine phosphatase. It was found that DEP-1 contains chondroitin sulfate chains, thus identifying DEP-1 as a proteoglycan. Furthermore, DEP-1 was found to interact with a heparan sulfate proteoglycan. No ligands have been identified for DEP-1. We have established a biacore-based assay for the identification of molecules interacting with the extracellular domain of DEP-1. A library of cell conditioned media was screened with the biacore assay. One of the samples was found to contain DEP-1 interacting molecules. Purification of the ligand has been initiated. In an attempt to identify modulators of DEP-1 activity, Matrigell™ , a preparation of extracellular matrix was investigated. Stimulation with Matrigel™ was found to increase the specific activity of DEP-1 through interactions between the extracellular domain of DEP-1 and Matrigel™ component(s).