Phospholipase C-β3 and -β1 Form Homodimers, but Not Heterodimers, through Catalytic and Carboxyl-Terminal Domains

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Abstract
Phospholipase C-β (PLC-β) isoenzymes are key effectors in G protein-coupled signaling pathways. Prior research suggests that some isoforms of PLC-β may exist and function as dimers. Using coimmunoprecipitation assays of differentially tagged PLC-β constructs and size-exclusion chromatography of native PLC-β, we observed homodimerization of PLC-β3 and PLC-β1 isoenzymes but failed to detect heterodimerization of these isoenzymes. Size-exclusion chromatography data suggest that PLC-β3 and PLC-β1 form higher affinity homodimers than PLC-β2. Evidence supportive of limited PLC-β monomer-homodimer equilibrium appears at ≤100 nM. Further assessment of homodimerization status by coimmunoprecipitation assays with differentially tagged PLC-β3 fragments demonstrated that at least two subdomains of PLC-β3 are involved in dimer formation, one in the catalytic X and Y domains and the other in the G protein-regulated carboxyl-terminal domain. In addition, we provide evidence consistent with the existence of PLC-β homodimers in a whole-cell context, using fluorescent protein-tagged constructs and microscopic fluorescence resonance energy transfer assays.