γ-Synuclein Interacts with Phospholipase Cβ2 to Modulate G Protein Activation

Phospholipase C beta 2 (PLC beta 2) is activated by G proteins and generates calcium signals in cells. PLC beta 2 is absent in normal breast tissue, but is highly expressed in breast tumors where its expression is correlated with the progression and migration of the tumor. This pattern of expression parallels the expression of the breast cancer specific gene protein 1 which is also known as gamma-synuclein. The cellular function of gamma-synuclein and the role it plays in proliferation are unknown. Here, we determined whether gamma-synuclein can interact with PLC beta 2 and affect its activity. Using co-immunprecitation and co-immunofluorescence, we find that in both benign and aggressive breast cancer cell lines gamma-synuclein and PLC beta 2 are associated. In solution, purified gamma-synuclein binds to PLC beta 2 with high affinity as measured by fluorescence methods. Protease digestion and mass spectrometry studies show that gamma-synuclein binds to a site on the C-terminus of PLC beta 2 that overlaps with the G alpha q binding site. Additionally, gamma-synuclein competes for Gaq association, but not for activators that bind to the N-terminus (i.e. Rac1 and G beta gamma). Binding of gamma-synuclein reduces the catalytic activity of PLC beta 2 by mechanism that involves inhibition of product release without affecting membrane interactions. Since activated G alpha q binds more strongly to PLC beta 2 than gamma-synuclein, addition of G alpha q(GTP gamma S) to the gamma-synuclein -PLC beta 2 complex allows for relief of enzyme inhibition along with concomitant activation. We also find that G beta gamma can reverse gamma-synuclein inhibition without dissociating the gamma-synuclein- PLC beta 2-complex. These studies point to a role of gamma-synuclein in promoting a more robust G protein activation of PLC beta 2.