UBC Theses and Dissertations
The function of protein tyrosine phosphatase alpha (PTPα) phosphorylation in integrin-mediated signaling Cheng, Suzanne Yuen Shan
The integrin signaling network involves over 180 components and regulates a wide range of biological activities including cell adhesion, survival and migration. However, the details of the molecular mechanisms that govern these cellular processes remain unclear. Since defective integrin signaling is often associated with diseases such as cancer, a precise understanding of the molecular mechanisms underlying integrin-mediated processes may provide novel insights for developing therapeutics against cancer. Protein tyrosine phosphatase alpha (PTPα) is a receptor-like PTP that activates Src family kinases (SFKs) upon integrin stimulation. In addition, its C-terminal tail Tyr789 phosphorylation, mediated by an active Src-FAK complex, promotes integrin-induced cell spreading, focal adhesion (FA) formation, and cell migration. I hypothesized that PTPα-phosphoTyr789 serves as an SH2-domain binding site to recruit other focal adhesion proteins to regulate cell migration. Studies involving re-expression of an unphosphorylatable mutant (Y789F) of PTPα in PTPα-null cells revealed that PTPα Tyr789 promotes FA localization and tyrosine phosphorylation of Cas, a key player in cell migration. Furthermore, BCAR3 was identified as a novel binding partner of PTPα-phosphoTyr789 that mediates Cas association with PTPα, in this way localizing Cas to FAs to promote downstream signaling to regulate cell migration. The adaptor protein Grb2 also interacts with PTPα-phosphoTyr789 but its role in association with PTPα is unknown. Using a Grb2-silencing approach, I discovered that Grb2 regulates integrin-induced PTPα Tyr789 phosphorylation via two distinct mechanisms: 1) Grb2 regulates FAK autophosphorylation and thus FAK/Src complex activation and 2) the SH2 and C-terminal SH3 domains of Grb2 mediate the formation of a PTPα-Grb2-FAK complex. Together, these roles of Grb2 promote Src-FAK-mediated PTPα tyrosine phosphorylation. In summary, my results reveal both upstream molecular mechanisms that regulate PTPα Tyr789 phosphorylation and downstream PTPα Tyr789-dependent events that regulate cell migration.
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