Determining selectivity of phosphoinositide-binding domains

A novel flow cytometric assay to quantify interactions between proteins and membrane lipids

A diverse set of experimental systems has been developed to probe protein-lipid interactions. These include measurements with the headgroups of membrane lipids in solution, immobilized membrane lipids, and analysis of protein binding to membrane lipids reconstituted in liposomes. Each of these methodologies has strengths but also substantial limitations. For example, measurements between proteins and lipid headgroups or with immobilized membrane lipids do not probe interactions in their natural environment, the lipid bilayer. The use of liposomes, however, was so far mostly restricted to biochemical flotation experiments that do not provide quantitative and/or kinetic data. Here, we present a fast and sensitive flow cytometric method to detect protein-lipid interactions. This technique allows for quantitative measurements of interactions between multiple fluorescently labeled proteins and membrane lipids reconstituted in lipid bilayers. The assay can be used to quantify binding efficiencies and to determine kinetic constants. The method is further characterized by a short sampling time of only a few seconds that allows for high-content screening procedures. Finally, using light scatter measurements, the described method also allows for monitoring changes of membrane curvature as well as tethering of liposomes evoked by binding of proteins.

TORC2 plasma membrane localization is essential for cell viability and restricted to a distinct domain

The conserved target of rapamycin (TOR) kinases regulate many aspects of cellular physiology. They exist in two distinct complexes, termed TOR complex 1 (TORC1) and TOR complex 2 (TORC2), that posses both overlapping and distinct components. TORC1 and TORC2 respond differently to the drug rapamycin and have different cellular functions: whereas the rapamycin-sensitive TORC1 controls many aspects of cell growth and has been characterized in great detail, the TOR complex 2 is less understood and regulates actin polymerization, cell polarity, and ceramide metabolism. How signaling specificity and discrimination between different input signals for the two kinase complexes is achieved is not understood. Here, we show that TORC1 and TORC2 have different localizations in Saccharomyces cerevisiae. TORC1 is localized exclusively to the vacuolar membrane, whereas TORC2 is localized dynamically in a previously unrecognized plasma membrane domain, which we term membrane compartment containing TORC2 (MCT). We find that plasma membrane localization of TORC2 is essential for viability and mediated by lipid binding of the C-terminal domain of the Avo1 subunit. From these data, we suggest that the TOR complexes are spatially separated to determine downstream signaling specificity and their responsiveness to different inputs.

Live cell imaging of phosphoinositides with expressed inositide binding protein domains

Inositol lipids and calcium signaling has been inseparable twins during the 1980s when the molecular details of phospholipase C-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and its Ca2+ mobilizing action were discovered. Since then, both the Ca2+ and inositol lipid signaling fields have hugely expanded and the tools allowing dissection of the finest details of their molecular organization also followed closely. Although phosphoinositides regulate many cell functions unrelated to Ca2+ signaling there are still many open questions even in the Ca2+ field that would benefit from single cell monitoring of PtdIns(4,5)P2 or InsP3 changes during agonist stimulation. This chapter is designed to provide practical guidance as well as some theoretical background on measurements of phosphoinositides in live cells using protein domain-GFP chimeras that could be also useful for people working on calcium signaling.

Mutations in the PX-SH3A linker of p47phox decouple PI(3,4)P2 binding from NADPH oxidase activation

NADPH oxidase is essential in the human innate immune response. p47 (phox), a cytosolic NADPH oxidase component, plays a regulatory role in the activation of NADPH oxidase. Our manipulation of p47 (phox) by mutation and amino acid deletion shows that the linker region between the PX and N-terminal SH3 domain plays a role in blocking the binding of the phosphoinositide 3,4-bisphosphate [PI(3,4)P2], a lipid second messenger generated upon neutrophil activation. Replacement of linker residues 151-158 with glycine alters NMR-measured spin lattice relaxation rates and sedimentation velocity compared to those of the wild-type protein, suggesting that the PX domain is released from its autoinhibited conformation. Liposome binding and surface plasmon resonance experiments confirm this result, showing that this mutant has a similar binding affinity for the isolated PX domain toward PI(3,4)P2. However, an in vitro NADPH oxidase activity assay reveals that this glycine mutant of the full-length protein greatly reduced NADPH oxidase activity upon activation even though it displayed PI(3,4)P2 binding activity comparable to that of the isolated PX domain. Our results highlight an active role of the PX-SH3 linker region in maintaining p47 (phox) in its fully autoinhibited form and demonstrate that binding of p47 (phox) to membrane phospholipids is mechanistically distinct from NADPH oxidase activation.

Membrane recognition by phospholipid-binding domains

Many different globular domains bind to the surfaces of cellular membranes, or to specific phospholipid components in these membranes, and this binding is often tightly regulated. Examples include pleckstrin homology and C2 domains, which are among the largest domain families in the human proteome. Crystal structures, binding studies and analyses of subcellular localization have provided much insight into how members of this diverse group of domains bind to membranes, what features they recognize and how binding is controlled. A full appreciation of these processes is crucial for understanding how protein localization and membrane topography and trafficking are regulated in cells.

A fluorescent sphingolipid binding domain peptide probe interacts with sphingolipids and cholesterol-dependent raft domains

We have designed a tagged probe [sphingolipid binding domain (SBD)] to facilitate the tracking of intracellular movements of sphingolipids in living neuronal cells. SBD is a small peptide consisting of the SBD of the amyloid precursor protein. It can be conjugated to a fluorophore of choice and exogenously applied to cells, thus allowing for in vivo imaging. Here, we present evidence to describe the characteristics of the SBD association with the plasma membrane. Our experiments demonstrate that SBD binds to isolated raft fractions from human neuroblastomas and insect neuronal cells. In protein-lipid overlay experiments, SBD interacts with a subset of glycosphingolipids and sphingomyelin, consistent with its raft association in neurons. We also provide evidence that SBD is taken up by neuronal cells in a cholesterol- and sphingolipid-dependent manner via detergent-resistant microdomains. Furthermore, using fluorescence correlation spectroscopy to assay the mobility of SBD in live cells, we show that SBD's behavior at the plasma membrane is similar to that of the previously described raft marker cholera toxin B, displaying both a fast and a slow component. Our data suggest that fluorescently tagged SBD can be used to investigate the dynamic nature of glycosphingolipid-rich detergent-resistant microdomains that are cholesterol-dependent.

ENTH domain proteins are cargo adaptors for multiple SNARE proteins at the TGN endosome

ENTH and ANTH domain proteins are involved in budding of clathrin-coated vesicles. SNAREs are fusogenic proteins that function in the targeting and fusion of transport vesicles. In mammalian and yeast cells, ENTH domain proteins (epsinR and Ent3p) interact with SNAREs of the vti1 family (Vti1b or Vti1p). This interaction indicates that ENTH proteins could function in cargo sorting, which prompted us to search for additional SNAREs as potential cargo for Ent3p and epsinR. We carried out specific yeast two-hybrid assays, which identified interactions between epsinR and the mammalian late endosomal SNAREs syntaxin 7 and syntaxin 8 as well as between Ent3p and the endosomal SNAREs Pep12p and Syn8p from yeast. Lack of Ent3p affected the trafficking of Pep12p. Ent3p binding to Pep12p required the FSD late endosomal sorting signal in Pep12p. Inactivation of the sorting signal had a similar effect to removal of Ent3p on Pep12p stability indicating that Ent3p acts as a cargo adaptor for Pep12p by binding to the sorting signal. As Vti1p, Pep12p and Syn8p participate in a SNARE complex whereas Vti1b, syntaxin 7 and syntaxin 8 are mammalian SNARE partners, we propose that ENTH domain proteins at the TGN-endosome are cargo adaptors for these endosomal SNAREs.

Molecular characteristics of phosphoinositide binding

Phosphoinositides in general and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2) or PIP(2)) in particular have been recently found to function as important regulators of ion channels. Yet, while specific residues have been identified that affect channel-PIP(2) interactions, the precise binding site of PIP(2) has not been determined in any case. In addition to binding ion channels, however, phosphoinositides interact with a plethora of other proteins, and in a number of cases, the crystallographic structures of the complexes have been determined. Based on a database of 25 complexed crystallographic structures, we have addressed the molecular characteristics of phosphoinositide binding to proteins. Implications to phosphoinositide binding to ion channels are also discussed.

Anandamide-induced Ca2+ elevation leading to p38 MAPK phosphorylation and subsequent cell death via apoptosis in human osteosarcoma cells

The effect of anandamide on human osteoblasts is unclear. This study examined the effect of anandamide on viability, apoptosis, mitogen-activated protein kinases (MAPKs) and Ca2+ levels in MG63 osteosarcoma cells. Anandamide at 50-200 microM decreased cell viability via apoptosis as demonstrated by propidium iodide staining and activation of caspase-3. Immunoblotting suggested that anandamide induced expression of ERK, JNK and p38 MAPK. Anandamide-induced cell death and apoptosis were reversed by SB203580, but not by PD98059 and SP600125, suggesting that anandamide's action was via p38 MAPK, but not via ERK and JNK. Anandamide at 1-100 microM induced [Ca2+]i increases. Removal of extracellular Ca2+ decreased the anandamide response, indicating that anandamide induced Ca2+ influx and Ca2+ release. Chelation of intracellular Ca2+ with BAPTA reversed anandamide-induced cell death and p38 MAPK phosphorylation. Collectively, in MG63 cells, anandamide induced [Ca2+]i increases which evoked p38 MAPK phosphorylation. This p38 MAPK phosphorylation subsequently activated caspase-3 leading to apoptosis.