The correlation between multidomain enzymes and multiple activation mechanisms--the case of phospholipase Cβ and its membrane interactions

Regulation of bifunctional proteins in cells: Lessons from the phospholipase Cβ/G protein pathway

Some proteins can serve multiple functions depending on different cellular conditions. An example of a bifunctional protein is inositide-specific mammalian phospholipase Cβ (PLCβ). PLCβ is activated by G proteins in response to hormones and neurotransmitters to increase intracellular calcium. Recently, alternate cellular function(s) of PLCβ have become uncovered. However, the conditions that allow these different functions to be operative are unclear. Like many mammalian proteins, PLCβ has a conserved catalytic core along with several regulatory domains. These domains modulate the intensity and duration of calcium signals in response to external sensory information, and allow this enzyme to inhibit protein translation in a noncatalytic manner. In this review, we first describe PLCβ's cellular functions and regulation of the switching between these functions, and then discuss the thermodynamic considerations that offer insight into how cells manage multiple and competitive associations allowing them to rapidly shift between functional states.

Phospholipase Cβ-TRAX Association Is Required for PC12 Cell Differentiation

When treated with nerve growth factor, PC12 cells will differentiate over the course of several days. Here, we have followed changes during differentiation in the cellular levels of phosphoinositide-specific phospholipase Cβ (PLCβ) and its activator, Gα_q, which together mediate Ca²⁺ release. We also followed changes in the level of the novel PLCβ binding partner TRAX (translin-associated factor X), which promotes RNA-induced gene silencing. We find that the level of PLCβ increases 4-fold within 24 h, whereas Gα_q increases only 1.4-fold, and this increase occurs ∼24 h later than PLCβ. Alternately, the level of TRAX remains constant over the 72 h tested. When PLCβ1 or TRAX is down-regulated, differentiation does not occur. The impact of PLCβ on differentiation appears independent of Gα_q as down-regulating Gα_q at constant PLCβ does not affect differentiation. Förster resonance energy transfer studies after PLCβ association with its partners indicate that PLCβ induced soon after nerve growth factor treatment associates with TRAX rather than Gα_q Functional measurements of Ca²⁺ signals to assess the activity of PLCβ-Gα_q complexes and measurements of the reversal of siRNA(GAPDH) to assess the activity of PLCβ-TRAX complexes additionally suggest that the newly synthesized PLCβ associates with TRAX to impact RNA-induced silencing. Taken together, our studies show that PLCβ, through its ability to bind TRAX and reverse RNA silencing of specific genes, plays a key role in switching PC12 cells to their differentiated state.

Regulation of PKD1-mediated Golgi to cell surface trafficking by Gαq subunits

BACKGROUND INFORMATION

Heterotrimeric GTP-binding proteins play a key role in cell trafficking regulation. Above all, specific Gβγ subunits have been shown to be a major component of a signal transduction pathway, which also involves phospholipases C (PLC), protein kinases C (PKC) and D (PKD), whose main function is to regulate transport between Golgi and plasma membrane. It was the involvement of PLC which led us to study the role of the other member of this G protein family, the α subunits, in the regulation of membrane fission at the Golgi apparatus.

RESULTS

Among constitutive active (QL) variants of different G protein α subunit sub-families, only GαqQL subunits were able to induce Golgi fragmentation, a phenotype that mainly reflects a membrane fission increase at this organelle. This phenotype was not observed with a GαqQL palmitoylation mutant, showing the need for a membrane-bounded subunit. Besides, GαqQL-dependent Golgi fission was blocked by specific PLC and PKC inhibitors, and in the presence of a PKD1-kinase dead variant. In addition, GαqQL was the only α subunit capable of inducing PKD1 phosphorylation. Finally, Vesicular Stomatitis Virus thermosensitive mutant glycoprotein (VSVG tsO45) transport assays have demonstrated that GαqQL acts directly on Golgi membranes to regulate trafficking between this organelle and plasma membrane.

CONCLUSIONS

All these results indicate Gαq subunits for the first time as a regulator of PKD-mediated intracellular trafficking between Golgi apparatus and plasma membrane, opening new perspectives in the understanding of internal trafficking regulation by external signals through G protein-coupled receptors.

γ-Synuclein interacts with phospholipase Cβ2 to modulate G protein activation

Phospholipase Cβ2 (PLC β2) is activated by G proteins and generates calcium signals in cells. PLCβ2 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 PLCβ2 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 PLCβ2 are associated. In solution, purified γ-synuclein binds to PLCβ2 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 PLCβ2 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 Gβγ). Binding of γ-synuclein reduces the catalytic activity of PLCβ2 by mechanism that involves inhibition of product release without affecting membrane interactions. Since activated Gαq binds more strongly to PLCβ2 than γ-synuclein, addition of Gαq(GTPγS) to the γ-synuclein -PLCβ2 complex allows for relief of enzyme inhibition along with concomitant activation. We also find that Gβγ can reverse γ-synuclein inhibition without dissociating the γ-synuclein- PLCβ2- complex. These studies point to a role of γ-synuclein in promoting a more robust G protein activation of PLCβ2.