ABSTRACT
Phospholipase Cß2 (PLC ß2) is activated by G proteins and generates calcium signals in cells. PLCß2 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 γ-synuclein. The cellular function of γ-synuclein and the role it plays in proliferation are unknown. Here, we determined whether γ-synuclein can interact with PLCß2 and affect its activity. Using co-immunprecitation and co-immunofluorescence, we find that in both benign and aggressive breast cancer cell lines γ-synuclein and PLCß2 are associated. In solution, purified γ-synuclein binds to PLCß2 with high affinity as measured by fluorescence methods. Protease digestion and mass spectrometry studies show that γ-synuclein binds to a site on the C-terminus of PLCß2 that overlaps with the Gαq binding site. Additionally, γ-synuclein competes for Gαq association, but not for activators that bind to the N-terminus (i.e. Rac1 and Gßγ). Binding of γ-synuclein reduces the catalytic activity of PLCß2 by mechanism that involves inhibition of product release without affecting membrane interactions. Since activated Gαq binds more strongly to PLCß2 than γ-synuclein, addition of Gαq(GTPγS) to the γ-synuclein -PLCß2 complex allows for relief of enzyme inhibition along with concomitant activation. We also find that Gßγ can reverse γ-synuclein inhibition without dissociating the γ-synuclein- PLCß2- complex. These studies point to a role of γ-synuclein in promoting a more robust G protein activation of PLCß2.
