Pleckstrin homology (PH) domains play a central role in a wide array of signaling pathways by binding second messenger lipids of the phosphatidylinositol phosphate (PIP) lipid family. A given type of PIP lipid is formed in a specific cellular membrane where it is generally a minor component of the bulk lipid mixture. For example, the signaling lipid PI(3,4,5)P 3 (or PIP 3 ) is generated primarily in the inner leaflet of the plasma membrane where it is believed to never exceed 0.02% of the bulk lipid. The present study focuses on the PH domain of the general receptor for phosphoinositides, isoform 1 (GRP1), which regulates the actin cytoskeleton in response to PIP 3 signals at the plasma membrane surface. The study systematically analyzes both the equilibrium and kinetic features of GRP1-PH domain binding to its PIP lipid target on a bilayer surface. Equilibrium binding measurements utilizing protein-to-membrane fluorescence resonance energy transfer (FRET) to detect GRP1-PH domain docking to membrane-bound PIP lipids confirm specific binding to PIP 3 . A novel FRET competitive binding measurement developed to quantitate docking affinity yields a K D of 50 ± 10 nM for GRP1-PH domain binding to membrane-bound PIP 3 in a physiological lipid mixture approximating the composition of the plasma membrane inner leaflet. This observed K D lies in a suitable range for regulation by physiological PIP 3 signals. Interestingly, the affinity of the interaction decreases at least 12-fold when the background anionic lipids phosphatidylserine (PS) and phosphatidylinositol (PI) are removed from the lipid mixture. Stopped-flow kinetic studies using protein-to-membrane FRET to monitor association and dissociation time courses reveal that this affinity decrease arises from a corresponding decrease in the on-rate for GRP1-PH domain docking with little or no change in the off-rate for domain dissociation from membrane-bound PIP 3 . Overall, these findings indicate that the PH domain interacts not only with its target lipid, but also with other features of the membrane surface. The results are consistent with a previously undescribed type of two-step search mechanism for lipid binding domains in which weak, nonspecific electrostatic interactions between the PH domain and background anionic lipids facilitate searching of the membrane surface for PIP 3 headgroups, thereby speeding the high-affinity, specific docking of the domain to its rare target lipid.Many significant events in cell signaling take place at membrane surfaces, particularly at the surface of the plasma membrane where receptors, channels, and signaling complexes make critical decisions to turn specific pathways on or off. An important class of membranebound second messengers that often play key roles in such decisions are the phosphatidylinositol phosphate lipids (PIP 1 lipids). Three of the most well characterized PIP signaling lipids in the plasma membrane are PI(3,4,5)P 3 , PI(3,4)P 2 , and PI(4,5)P 2 . These † Support provided by NIH Grant GM R01-63235 (to J.J...

We explored the mechanism of action of CD39 antibodies that inhibit ectoenzyme CD39 conversion of extracellular ATP (eATP) to AMP and thus potentially augment eATP-P2-mediated proinfl ammatory responses. Using syngeneic and humanized tumor models, we contrast the potency and mechanism of anti-CD39 mAbs with other agents targeting the adenosinergic pathway. We demonstrate the critical importance of an eATP-P2X7-ASC-NALP3infl ammasome-IL18 pathway in the antitumor activity mediated by CD39 enzyme blockade, rather than simply reducing adenosine as mechanism of action. Effi cacy of anti-CD39 activity was underpinned by CD39 and P2X7 coexpression on intratumor myeloid subsets, an early signature of macrophage depletion, and active IL18 release that facilitated the signifi cant expansion of intratumor effector T cells. More importantly, anti-CD39 facilitated infi ltration into T cell-poor tumors and rescued anti-PD-1 resistance. Anti-human CD39 enhanced human T-cell proliferation and Th1 cytokine production and suppressed human B-cell lymphoma in the context of autologous Epstein-Barr virus-specifi c T-cell transfer. SIGNIFICANCE :Overall, these data describe a potent and novel mechanism of action of antibodies that block mouse or human CD39, triggering an eATP-P2X7-infl ammasome-IL18 axis that reduces intratumor macrophage number, enhances intratumor T-cell effector function, overcomes anti-PD-1 resistance, and potentially enhances the effi cacy of adoptive T-cell transfer.

The C2 domain is a ubiquitous, conserved protein signaling motif widely found in eukaryotic signaling proteins. Although considerable functional diversity exists, most C2 domains are activated by Ca 2+ binding and then dock to a specific cellular membrane. The C2 domains of protein kinase Cα (PKCα) and cytosolic phospholipase A 2 α (cPLA 2 α), for example, are known to dock to different membrane surfaces during an intracellular Ca 2+ signal. Ca 2+ activation targets the PKCα C2 domain to the plasma membrane and the cPLA 2 α C2 domain to the internal membranes, with no detectable spatial overlap. It is crucial to determine how such targeting specificity is achieved at physiological bulk Ca 2+ concentrations that during a typical signaling event rarely exceed 1 μM. For the isolated PKCα C2 domain in the presence of physiological Ca 2+ levels, the target lipids phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PIP 2 ) are together sufficient to recruit the PKCα C2 domain to a lipid mixture mimicking the plasma membrane inner leaflet. For the cPLA 2 α C2 domain, the target lipid phosphatidylcholine (PC) appears to be sufficient to drive membrane targeting to an internal membrane mimic at physiological Ca 2+ levels, although the results do not rule out a second, unknown target molecule. Stopped-flow kinetic studies provide additional information about the fundamental molecular events that occur during Ca 2+ -activated membrane docking. In principle, C2 domain-directed intracellular targeting, which requires coincidence detection of multiple signals (Ca 2+ and one or more target lipids), can exhibit two different mechanisms: messenger-activated target affinity (MATA) and target-activated messenger affinity (TAMA). The C2 domains studied here both utilize the TAMA mechanism, in which the C2 domain Ca 2+ affinity is too low to be activated by physiological Ca 2+ signals in most regions of the cell. Only when the C2 domain nears its target membrane, which provides a high local concentration of target lipid, is the effective Ca 2+ affinity increased by the coupled binding equilibrium to a level that enables substantial Ca 2+ activation and target docking. Overall, the findings emphasize the importance of using physiological ligand concentrations in targeting studies because super-physiological concentrations can drive docking interactions even when an important targeting molecule is missing.Many signaling pathways are regulated by signaling lipids, membrane proteins, or membranebound complexes associated with the plasma or internal cell membranes. Such membraneassociated signaling components control essential processes, such as cellular movement, growth, gene regulation, metabolism, hormone release, and inflammation. One of the most common regulatory elements in membrane-associated signaling pathways is the C2 domain, a ubiquitous, conserved signaling motif recognized in over 200 mammalian proteins (1). Structurally, the C2 domain motif comprises eight antiparallel β-strands assembled in a β- † F...

Many patients with diabetes mellitus (both type 1 and type 2) require therapy to maintain normal fasting glucose levels. To develop a novel treatment for these individuals, we used phage display technology to target the insulin receptor (INSR) complexed with insulin and identified a high affinity, allosteric, human monoclonal antibody, XMetA, which mimicked the glucoregulatory, but not the mitogenic, actions of insulin. Biophysical studies with cultured cells expressing human INSR demonstrated that XMetA acted allosterically and did not compete with insulin for binding to its receptor. XMetA was found to function as a specific partial agonist of INSR, eliciting tyrosine phosphorylation of INSR but not the IGF-IR. Although this antibody activated metabolic signaling, leading to enhanced glucose uptake, it neither activated Erk nor induced proliferation of cancer cells. In an insulin resistant, insulinopenic model of diabetes, XMetA markedly reduced elevated fasting blood glucose and normalized glucose tolerance. After 6 weeks, significant improvements in HbA1c, dyslipidemia, and other manifestations of diabetes were observed. It is noteworthy that hypoglycemia and weight gain were not observed during these studies. These studies indicate, therefore, that allosteric monoclonal antibodies have the potential to be novel, ultra-long acting, agents for the regulation of hyperglycemia in diabetes.

A novel photoreactive analog of cholesterol, 3alpha-(4-azido-3-[125I]iodosalicylic)-cholest-5-ene ([125I]azido-cholesterol), was used to label both native acetylcholine receptor (AChR)-rich membranes from Torpedo californica and affinity-purified Torpedo AChRs reconstituted into lipid vesicles. In both cases all four AChR subunits incorporated [125I]azido-cholesterol on an equal molar basis and neither the pattern nor the extent of labeling was affected by the presence of the agonist carbamylcholine. Labeled regions in each of the AChR subunits were initially mapped by Staphylococcus aureus V8 protease digestion to large fragments which contain the AChR transmembrane segments. Sites of [125I]azido-cholesterol incorporation were further mapped by exhaustive tryptic digestion of the V8 protease subunit fragments alphaV8-20 (alphaSer-173-Glu-338), alphaV8-10 (alphaAsn-339-Gly-439), and gammaV8-14 (gammaLeu-373-Pro-489). The digests were separated by reverse-phase high-performance liquid chromatography and labeled peptides identified by amino-terminal sequence analysis. [125I]Azido-cholesterol labeling was localized to peptides that contain almost exclusively the alpha-M4, alpha-M1 and gamma-M4 membrane spanning segments. These results establish that the binding domain for cholesterol is at the lipid-protein interface of the AChR.