Dynamics of allosteric regulation of the phospholipase C-γ isozymes upon recruitment to membranes

Numerous receptor tyrosine kinases and immune receptors activate phospholipase C-γ (PLC-γ) isozymes at membranes to control diverse cellular processes including phagocytosis, migration, proliferation, and differentiation. The molecular details of this process are not well understood. Using hydrogen-deuterium exchange mass spectrometry, we show that PLC-γ1 is relatively inert to lipid vesicles that contain its substrate, phosphatidylinositol 4,5-bisphosphate (PIP₂), unless first bound to the kinase domain of the fibroblast growth factor receptor (FGFR1). Exchange occurs throughout PLC-γ1 and is exaggerated in PLC-γ1 containing an oncogenic substitution (D1165H) that allosterically activates the lipase. These data support a model whereby initial complex formation shifts the conformational equilibrium of PLC-γ1 to favor activation. This receptor-induced priming of PLC-γ1 also explains the capacity of a kinase-inactive fragment of FGFR1 to modestly enhance the lipase activity of PLC-γ1 operating on lipid vesicles but not a soluble analog of PIP₂ and highlights potential cooperativity between receptor engagement and membrane proximity. Priming is expected to be greatly enhanced for receptors embedded in membranes and nearly universal for the myriad of receptors and co-receptors that bind the PLC-γ isozymes.

A High-Throughput Assay to Identify Allosteric Inhibitors of the PLC-γ Isozymes Operating at Membranes

The two phospholipase C-γ (PLC-γ) isozymes are major signaling hubs and emerging therapeutic targets for various diseases, yet there are no selective inhibitors for these enzymes. We have developed a high-throughput, liposome-based assay that features XY-69, a fluorogenic, membrane-associated reporter for mammalian PLC isozymes. The assay was validated using a pilot screen of the Library of Pharmacologically Active Compounds 1280 (LOPAC₁₂₈₀) in 384-well format; it is highly reproducible and has the potential to capture both orthosteric and allosteric inhibitors. Selected hit compounds were confirmed with secondary assays, and further profiling led to the interesting discovery that adenosine triphosphate potently inhibits the PLC-γ isozymes through noncompetitive inhibition, raising the intriguing possibility of endogenous, nucleotide-dependent regulation of these phospholipases. These results highlight the merit of the assay platform for large scale screening of chemical libraries to identify allosteric modulators of the PLC-γ isozymes as chemical probes and for drug discovery.

Structural basis for the activation of PLC-γ isozymes by phosphorylation and cancer-associated mutations

Direct activation of the human phospholipase C-γ isozymes (PLC-γ1, -γ2) by tyrosine phosphorylation is fundamental to the control of diverse biological processes, including chemotaxis, platelet aggregation, and adaptive immunity. In turn, aberrant activation of PLC-γ1 and PLC-γ2 is implicated in inflammation, autoimmunity, and cancer. Although structures of isolated domains from PLC-γ isozymes are available, these structures are insufficient to define how release of basal autoinhibition is coupled to phosphorylation-dependent enzyme activation. Here, we describe the first high-resolution structure of a full-length PLC-γ isozyme and use it to underpin a detailed model of their membrane-dependent regulation. Notably, an interlinked set of regulatory domains integrates basal autoinhibition, tyrosine kinase engagement, and additional scaffolding functions with the phosphorylation-dependent, allosteric control of phospholipase activation. The model also explains why mutant forms of the PLC-γ isozymes found in several cancers have a wide spectrum of activities, and highlights how these activities are tuned during disease.

RAS ubiquitylation modulates effector interactions

RAS proteins function as molecular switches that regulate cellular growth by cycling between active GTP- and inactive GDP bound states. While RAS activity is modulated by factors (guanine nucleotide exchange and GTPase activating proteins) that control levels of active Ras-GTP, RAS proteins also undergo a number of post-translational modifications that regulate their function. One such modification is ubiquitylation. Monoubiquitylation of KRAS at lysine 147 (mUbRAS) enhances Ras activation and promotes signaling through the RAF and Phosphoinositide 3-Kinase (PI3K) signaling pathways. We have previously shown that mUbRAS leads to activation of RAS through a defect in GTPase activating protein (GAP) mediated downregulation, similar to the action of most oncogenic mutations. Consistent with these findings, we now show that mUbRASimpairsRAS binding to the p120 GAP catalytic domain. Mutations in activated G12V RAS that prevent ubiquitylaton at 147 show a decrease in tumorigenesis, suggesting that in addition to activating KRAS, monoubiquitylation at this site may promote downstream signaling and transformation. To investigate whether mUbRAS alters RAS effector interactions, we chemically ubiquitylated KRAS at residue 147 and characterized binding of mUbRAS to RAS binding domains (RBDs) from three distinct downstream effectors that play key roles in RAS-mediated transformation. Results from these studies show a decrease in binding of mUbRAS (7-10-fold) relative to the CRAF RAS Binding Domain (RBD), the catalytic subunit of Phosphoinositide 3-Kinase catalytic gamma (PI3Kcγ) and RALGDS RBD. Intriguingly, we find that mUbRAS shows greatly enhanced (> 40-fold) binding to the CRAF RBD when bound to GDP. These findings, taken together, suggest that mUbRASmay promoteactivation of RAS through a GAP defect, and facilitate RAF association and MAPK signaling in a nucleotide independent manner.

Methylation specific targeting of a chromatin remodeling complex from sponges to humans

DNA cytosine methylation and methyl-cytosine binding domain (MBD) containing proteins are found throughout all vertebrate species studied to date. However, both the presence of DNA methylation and pattern of methylation varies among invertebrate species. Invertebrates generally have only a single MBD protein, MBD2/3, that does not always contain appropriate residues for selectively binding methylated DNA. Therefore, we sought to determine whether sponges, one of the most ancient extant metazoan lineages, possess an MBD2/3 capable of recognizing methylated DNA and recruiting the associated nucleosome remodeling and deacetylase (NuRD) complex. We find that Ephydatia muelleri has genes for each of the NuRD core components including an EmMBD2/3 that selectively binds methylated DNA. NMR analyses reveal a remarkably conserved binding mode, showing almost identical chemical shift changes between binding to methylated and unmethylated CpG dinucleotides. In addition, we find that EmMBD2/3 and EmGATAD2A/B proteins form a coiled-coil interaction known to be critical for the formation of NuRD. Finally, we show that knockdown of EmMBD2/3 expression disrupts normal cellular architecture and development of E. muelleri. These data support a model in which the MBD2/3 methylation-dependent functional role emerged with the earliest multicellular organisms and has been maintained to varying degrees across animal evolution.