Debasis Manna

Mechanistic Basis of Differential Cellular Responses of Phosphatidylinositol 3,4-Bisphosphate- and Phosphatidylinositol 3,4,5-Trisphosphate-binding Pleckstrin Homology Domains

Phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) are lipid second messengers that regulate various cellular processes by recruiting a wide range of downstream effector proteins to membranes. Several pleckstrin homology (PH) domains have been reported to interact with PtdIns(3,4)P2 and PtdIns(3,4,5)P3. To understand how these PH domains differentially respond to PtdIns(3,4)P2 and PtdIns(3,4,5)P3 signals, we quantitatively determined the PtdIns(3,4)P2 and PtdIns(3,4,5)P3 binding properties of several PH domains, including Akt, ARNO, Btk, DAPP1, Grp1, and C-terminal TAPP1 PH domains by surface plasmon resonance and monolayer penetration analyses. The measurements revealed that these PH domains have significant different phosphoinositide specificities and affinities. Btk-PH and TAPP1-PH showed genuine PtdIns(3,4,5)P3 and PtdIns(3,4)P2 specificities, respectively, whereas other PH domains exhibited less pronounced specificities. Also, the PH domains showed different degrees of membrane penetration, which greatly affected the kinetics of their membrane dissociation. Mutational studies showed that the presence of two proximal hydrophobic residues on the membrane-binding surface of the PH domain is important for membrane penetration and sustained membrane residence. When NIH 3T3 cells were stimulated with platelet-derived growth factor to generate PtdIns(3,4,5)P3, reversible translocation of Btk-PH, Grp1-PH, ARNO-PH, DAPP1-PH, and its L177A mutant to the plasma membrane was consistent with their in vitro membrane binding properties. Collectively, these studies provide new insight into how various PH domains would differentially respond to cellular PtdIns(3,4)P2 and PtdIns(3,4,5)P3 signals.

Differential Roles of Phosphatidylserine, PtdIns(4,5)P2, and PtdIns(3,4,5)P3 in Plasma Membrane Targeting of C2 Domains MOLECULAR DYNAMICS SIMULATION, MEMBRANE BINDING, AND CELL TRANSLOCATION STUDIES OF THE PKCα C2 Domain

Many cytosolic proteins are recruited to the plasma membrane (PM) during cell signaling and other cellular processes. Recent reports have indicated that phosphatidylserine (PS), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) that are present in the PM play important roles for their specific PM recruitment. To systematically analyze how these lipids mediate PM targeting of cellular proteins, we performed biophysical, computational, and cell studies of the Ca2+-dependent C2 domain of protein kinase Cα (PKCα) that is known to bind PS and phosphoinositides. In vitro membrane binding measurements by surface plasmon resonance analysis show that PKCα-C2 nonspecifically binds phosphoinositides, including PtdIns(4,5)P2 and PtdIns(3,4,5)P3, but that PS and Ca2+ binding is prerequisite for productive phosphoinositide binding. PtdIns(4,5)P2 or PtdIns(3,4,5)P3 augments the Ca2+- and PS-dependent membrane binding of PKCα-C2 by slowing its membrane dissociation. Molecular dynamics simulations also support that Ca2+-dependent PS binding is essential for membrane interactions of PKCα-C2. PtdIns(4,5)P2 alone cannot drive the membrane attachment of the domain but further stabilizes the Ca2+- and PS-dependent membrane binding. When the fluorescence protein-tagged PKCα-C2 was expressed in NIH-3T3 cells, mutations of phosphoinositide-binding residues or depletion of PtdIns(4,5)P2 and/or PtdIns(3,4,5)P3 from PM did not significantly affect the PM association of the domain but accelerated its dissociation from PM. Also, local synthesis of PtdIns(4,5)P2 or PtdIns(3,4,5)P3 at the PM slowed membrane dissociation of PKCα-C2. Collectively, these studies show that PtdIns(4,5)P2 and PtdIns(3,4,5)P3 augment the Ca2+- and PS-dependent membrane binding of PKCα-C2 by elongating the membrane residence of the domain but cannot drive the PM recruitment of PKCα-C2. These studies also suggest that effective PM recruitment of many cellular proteins may require synergistic actions of PS and phosphoinositides.

Unexpected Complexity in the Mechanisms That Target Assembly of the Spectrin Cytoskeleton

The spectrin cytoskeleton assembles within discrete regions of the plasma membrane in a wide range of animal cell types. Although recent studies carried out in vertebrate systems indicate that spectrin assembly occurs indirectly through the adapter protein ankyrin, recent studies in Drosophila have established that spectrin can also assemble through a direct ankyrin-independent mechanism. Here we tested specific regions of the spectrin molecule for a role in polarized assembly and function. First, we tested mutant β-spectrins lacking ankyrin binding activity and/or the COOH-terminal pleckstrin homology (PH) domain for their assembly competence in midgut, salivary gland, and larval brain. Remarkably, three different assembly mechanisms operate in these three cell types: 1) neither site was required for assembly in salivary gland; 2) only the PH domain was required in midgut copper cells; and 3) either one of the two sites was sufficient for spectrin assembly in larval brain. Further characterization of the PH domain revealed that it binds strongly to lipid mixtures containing phosphatidylinositol 4,5-bisphosphate (PIP2) but not phosphatidylinositol 3,4,5-trisphosphate. A K8Q mutation in the lipid binding region of the PH domain eliminated the PIP2 interaction in vitro, yet the mutant protein retained full biological function in vivo. Reporter gene studies revealed that PIP2 and the spectrin PH domain codistribute with one another in cells but not with authentic wild type αβ-spectrin. Thus, it appears that the PH domain imparts membrane targeting activity through a second mechanism that takes precedence over its PIP2 binding activity.

Microplate-Based Characterization of Protein-Phosphoinositide Binding Interactions Using a Synthetic Biotinylated Headgroup Analogue

Membrane lipids act as important regulators of a litany of important physiological and pathophysiological events. Many of them act as site-specific ligands for cytosolic proteins in binding events that recruit receptors to the cell surface and control both protein function and subcellular localization. Phosphatidylinositol phosphates (PIP(n)s) are a family of signaling lipids that regulate numerous cellular processes by interacting with a myriad of protein binding modules. Characterization of PIP(n)-binding proteins has been hampered by the lack of a rapid and convenient quantitative assay. Herein, microplate-based detection is presented as an effective approach to characterizing protein-PIP(n) binding interactions at the molecular level. With this assay, the binding of proteins to isolated PIP(n) headgroups is detected with high sensitivity using a platform that is amenable to high-throughput screening. In the studies described herein, biotinylated PI-(4,5)-P(2) headgroup analogue 1 was designed, synthesized, and immobilized onto 96-well streptavidin-coated microplates to study receptor binding. This assay was used to characterize the binding of the PH domain of beta-spectrin to this headgroup. The high affinity interaction that was detected for surface association (K(d, surf) = 6 nM +/- 3), demonstrates that receptor binding modules can form high affinity interactions with lipid headgroups outside of a membrane environment. The results also indicate the feasibility of the assay for rapid characterization of PIP(n)-binding proteins as well as the promise for high-throughput analysis of protein-PIP(n) binding interactions. Finally, this assay was also employed to characterize the inhibition of the binding of receptors to the PIP(n)-derivatized microplates using solution phase competitors. This showcases the viability of this assay for rapid screening of inhibitors of PIP(n)-binding proteins.

Alkyl cinnamates as regulator for the C1 domain of protein kinase C isoforms.

The protein kinase C (PKC) family of serine/threonine kinases is an attractive drug target for the treatment of cancer and other diseases. Natural product curcumin is known to interact with PKC isoforms through the C1 domain and modulate PKC activity. The reported results demonstrate that the symmetric curcumin molecule might act as two separate units during its recognition of C1 domains. To understand the importance of the two halves of curcumin in PKC binding and to develop effective PKC regulators, we synthesized a series of alkyl cinnamates (1-8), characterized absorption and fluorescence properties and measured binding affinities with the C1b subdomains of PKC isoforms. The binding parameters of the monomeric compounds and liposomes containing compounds confirmed their interaction with the C1b subdomains of PKCδ and PKCθ. The molecular docking analysis with PKCδ and PKCθ C1b subdomains revealed that the alkyl cinnamates form hydrogen bond with the backbone of the protein at the same binding site as that of diacylglycerol and phorbol esters. The results show that the alkyl cinnamates bind to the activator binding site of PKCs and both methoxy and hydroxyl groups play important roles in the binding process.

Zn(OTf) 2 ‑Promoted Chemoselective Esterification of Hydroxyl Group Bearing Carboxylic Acids

Selective esterification of aliphatic and aromatic carboxylic acids with various alcohols is studied using triphenylphosphine, I2, and a catalytic amount of Zn(OTf)2. Use of this catalyst allows the formation of esters at a faster rate with good to excellent yield by activating the in situ generated acyloxyphosphonium ion intermediate. During the esterification process, both their aromatic and aliphatic hydroxyl groups are fully preserved from transesterification. The results show that the bulkiness and the reactivity of this doubly activated intermediate III control the selectivity and the rate of the reaction, respectively. The method is also useful for direct amidation reactions.

Effects of ortho substituent groups of protocatechualdehyde derivatives on binding to the C1 domain of novel protein kinase C.

Diacylglycerol (DAG) regulates a broad range of cellular functions including tumor promotion, apoptosis, differentiation, and growth. Thus, the DAG-responsive C1 domain of protein kinase C (PKC) isoenzymes is considered to be an attractive drug target for the treatment of cancer and other diseases. To develop effective PKC regulators, we conveniently synthesized (hydroxymethyl)phenyl ester analogues targeted to the DAG binding site within the C1 domain. Biophysical studies and molecular docking analysis showed that the hydroxymethyl group, hydrophobic side chains, and acyl group at the ortho position are essential for their interactions with the C1-domain backbone. Modifications of these groups showed diminished binding to the C1 domain. The active (hydroxymethyl)phenyl ester analogues showed more than 5-fold stronger binding affinity for the C1 domain than DAG. Therefore, our findings reveal that (hydroxymethyl)phenyl ester analogues represent an attractive group of C1-domain ligands that can be further structurally modified to improve their binding and activity.

Physicochemical characterization of diacyltetrol-based lipids consisting of both diacylglycerol and phospholipid headgroups

We describe the synthesis of diacyltetrol-based hybrid lipids in which one of the hydroxymethyl groups is modified with an anionic phospholipid headgroup. The hybrid lipids form a monolayer at the air–water interface. In aqueous solution, these lipids form stable liposomes that exhibit a negative surface potential across a wide pH range. The liposomes aggregate in the presence of Ca2+ ions and release encapsulated cationic reporter rhodamine 6G (R6G) at a faster rate than anionic reporter carboxyfluorescein (CF). The hybrid lipids strongly interact with the C1b subdomain of the protein kinase C (PKC)-θ isoform. These new lipids structurally mimic diacylglycerol and conventional phospholipids, and provide an opportunity to explore their physicochemical properties.

Real-Time Cell Assays of Phospholipase A2s Using Fluorogenic Phospholipids

Phospholipase A(2)s (PLA(2)s) are a superfamily of enzymes involved in production of a wide variety of lipid mediators, including arachidonic acid, lysophospholipids, platelet activation factor, and eicosanoids. Fluorescence-based, real-time cellular activity assays for PLA(2)s have been developed as a tool for studying the function and spatiotemporal regulation of PLA(2)s. Recent progress in fluorogenic phospholipid design, genetic methods, and multiphoton multichannel microscopy allows simultaneous and continuous measurement of cellular localization and activity of PLA(2) in an isoform-selective manner. These assays should aid in elucidating the physiological roles and regulatory mechanisms of diverse PLA(2) isoforms, developing isoform-specific PLA(2) inhibitors, and analyzing lipidomics data of PLA(2) products.

3‐Amino‐4‐aminoximidofurazan derivatives: small molecules possessing antimicrobial and antibiofilm activity against Staphylococcus aureus and Pseudomonas aeruginosa

The therapeutic treatment of microbial infections involving biofilm becomes quite challenging because of its increasing antibiotic resistance capacities. Towards this direction, in the present study we have evaluated the antibiofilm property of synthesized 3‐amino‐4‐aminoximidofurazan compounds having polyamine skeleton. These derivatives were synthesized by incorporating furazan and biguanide moieties.