Peptides that mimic the pseudosubstrate region of protein kinase C bind to acidic lipids in membranes

Into the fold: advances in understanding aPKC membrane dynamics

Atypical protein kinase Cs (aPKCs) are part of the PKC family of protein kinases and are atypical because they don't respond to the canonical PKC activators diacylglycerol (DAG) and Ca2+. They are central to the organization of polarized cells and are deregulated in several cancers. aPKC recruitment to the plasma membrane compartment is crucial to their encounter with substrates associated with polarizing functions. However, in contrast with other PKCs, the mechanism by which atypical PKCs are recruited there has remained elusive until recently. Here, we bring aPKC into the fold, summarizing recent reports on the direct recruitment of aPKC to membranes, providing insight into seemingly discrepant findings and integrating them with existing literature.

Control of atypical PKCι membrane dissociation by tyrosine phosphorylation within a PB1-C1 interdomain interface

Atypical PKCs are cell polarity kinases that operate at the plasma membrane where they function within multiple molecular complexes to contribute to the establishment and maintenance of polarity. In contrast to the classical and novel PKCs, atypical PKCs do not respond to diacylglycerol cues to bind the membrane compartment. Until recently it was not clear how aPKCs are recruited; whether aPKCs can directly interact with membranes or whether they are dependent on other protein interactors to do so. Two recent studies identified the pseudo-substrate region and the C1 domain as direct membrane interaction modules, however their relative importance and coupling are unknown. We combined molecular modelling and functional assays to show that the regulatory module of aPKCι, comprising the PB1 pseudo-substrate and C1 domains, forms a cooperative and spatially continuous invariant membrane interaction platform. Furthermore, we show the coordinated orientation of membrane-binding elements within the regulatory module requires a key PB1-C1 interfacial β-strand (BSL). We show this element contains a highly conserved Tyr residue that can be phosphorylated and that negatively regulates the integrity of the regulatory module, leading to membrane release. We thus expose a hitherto unknown regulatory mechanism of aPKCι membrane binding and release during cell polarization.

Contributing mechanisms underlying desensitization of cholecystokinin-induced activation of primary nodose ganglia neurons

Cholecystokinin (CCK) is a gut-derived peptide that potently promotes satiety and facilitates gastric function in part by activating G protein-coupled CCK1 receptors on primary vagal afferent neurons. CCK signaling is dynamic and rapidly desensitizes, due to decreases in either receptor function and the resulting signal cascade, ion channel effectors, or both. Here we report a decay-time analytical approach using fluorescent calcium imaging that relates peak and steady-state calcium responses in dissociated vagal afferent neurons, enabling discrimination between receptor and ion channel effector functions. We found desensitization of CCK-induced activation was predictable, consistent across cells, and strongly concentration dependent. The decay-time constant (tau) was inversely proportional to CCK concentration, apparently reflecting the extent of receptor activation. To test this possibility, we directly manipulated the ion channel effector(s) with either decreased bath calcium or the broad-spectrum pore blocker ruthenium red. Conductance inhibition diminished the magnitude of the CCK responses without altering decay kinetics, confirming changes in tau reflect changes in receptor function selectively. Next, we investigated the contributions of the PKC and PKA signaling cascades on the magnitude and decay-time constants of CCK calcium responses. While inhibition of either PKC or PKA increased CCK calcium response magnitude, only general PKC inhibition significantly decreased the decay-time constant. These findings suggest that PKC alters CCK receptor signaling dynamics, while PKA alters the ion channel effector of the CCK response. This analytical approach should prove useful in understanding receptor/effector changes underlying acute desensitization of G-protein coupled signaling and provide insight into CCK receptor dynamics.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Phosphoinositides and Membrane Targeting in Cell Polarity

Selective enrichment of the polyphosphoinositides (PPIn), such as PtdIns(4,5)P2 and PtdIns4P, helps to determine the identity of the plasma membrane (PM) and regulates many aspects of cell biology through a vast number of protein effectors. Polarity proteins had long been assumed to be non-PPIn-binding proteins that mainly associate with PM/cell cortex through their extensive protein-protein interaction network. However, recent studies began to reveal that several key polarity proteins electrostatically bind to PPIn through their positively charged protein domains or structures and such PPIn-binding property is essential for their direct and specific attachment to PM. Although the physical nature of the charge-based PPIn binding appears to be simple and nonspecific, it serves as an elegant mechanism that can be efficiently and specifically regulated for achieving polarized PM targeting of polarity proteins. As an unexpected consequence, subcellular localization of PPIn-binding polarity proteins are also subject to regulations by physiological conditions such as hypoxia and ischemia that acutely and reversibly depletes PPIn from PM.

Dynamics and Membrane Interactions of Protein Kinase C

Protein kinase C (PKC) is a family of Ser/Thr kinases that regulate a multitude of cellular processes through participation in the phosphoinositide signaling pathway. Significant research efforts have been directed at understanding the structure, function, and regulatory modes of the enzyme since its discovery and identification as the first receptor for tumor-promoting phorbol esters. The activation of PKC involves a transition from the cytosolic autoinhibited latent form to the membrane-associated active form. The membrane recruitment step is accompanied by the conformational rearrangement of the enzyme, which relieves autoinhibitory interactions and thereby allows PKC to phosphorylate its targets. The multidomain structure and intrinsic flexibility of PKC present remarkable challenges and opportunities for the biophysical and structural biology studies of this class of enzymes and their interactions with membranes, the major focus of this Current Topic. I will highlight the recent advances in the field, outline the current challenges, and identify areas where biophysics and structural biology approaches can provide insight into the isoenzyme-specific regulation of PKC activity.

Protein Kinase C (PKC)ζ Pseudosubstrate Inhibitor Peptide Promiscuously Binds PKC Family Isoforms and Disrupts Conventional PKC Targeting and Translocation

PKMζ is generated via an alternative transcriptional start site in the atypical protein kinase C (PKC)ζ isoform, which removes N-terminal regulatory elements, including the inhibitory pseudosubstrate domain, consequently rendering the kinase constitutively active. Persistent PKMζ activity has been proposed as a molecular mechanism for the long-term maintenance of synaptic plasticity underlying some forms of memory. Many studies supporting a role for PKMζ in synaptic plasticity and memory have relied on the PKCζ pseudosubstrate-derived ζ-inhibitory peptide (ZIP). However, recent studies have demonstrated that ZIP-induced impairments to synaptic plasticity and memory occur even in the absence of PKCζ, suggesting that ZIP exerts its actions via additional cellular targets. In this study, we demonstrated that ZIP interacts with conventional and novel PKC, in addition to atypical PKC isoforms. Moreover, when brain abundance of each PKC isoform and affinity for ZIP are taken into account, the signaling capacity of ZIP-responsive pools of conventional and novel PKCs may match or exceed that for atypical PKCs. Pseudosubstrate-derived peptides, like ZIP, are thought to exert their cellular action primarily by inhibiting PKC catalytic activity; however, the ZIP-sensitive catalytic core of PKC is known to participate in the enzyme's subcellular targeting, suggesting an additional mode of ZIP action. Indeed, we have demonstrated that ZIP potently disrupts PKCα interaction with the PKC-targeting protein A-kinase anchoring protein (AKAP) 79 and interferes with ionomycin-induced translocation of conventional PKC to the plasma membrane. Thus, ZIP exhibits broad-spectrum action toward the PKC family of enzymes, and this action may contribute to its unique ability to impair memory.

The cytosolic splicing variant of NELL2 inhibits PKCβ1 in glial cells

NELL2 is an abundant glycoprotein containing EGF-like domain in the neural tissues where it has multiple physiological functions by interacting with protein kinase C (PKC). There are two different splicing variant forms of NELL2 identified so far. One is secreted NELL2 (sNELL2) which is a neuron-specific variant and the other is cytosolic NELL2 (cNELL2) which is non-secreted splicing variant of NELL2. Although cNELL2 structure was well characterized, the expression pattern or the cellular function of cNELL2 is not fully determined. In this study, we found that cNELL2 specifically interacts with PKCβ isotypes and inhibits PKCβ1 through direct binding to the N-terminal pseudosubstrate domain of PKCβ1. Here, we also demonstrate that cNELL2 is predominantly expressed and has inhibitory effects on the PKC downstream signaling pathways in astrocytes thereby establishing cNELL2 as an endogenous inhibitor of PKCβ1 in glia.

Investigation of membrane penetration depth and interactions of the amino-terminal domain of huntingtin: refined analysis by tryptophan fluorescence measurement

The membrane-association properties of the amino-terminal domain of huntingtin are accompanied by subcellular redistribution of the protein in cellular compartments. In this study we used tryptophan substitution of amino-acid residues at different positions of the huntingtin 1-17 domain (Htt17) to precisely determine, for the first time, the depth of penetration of the peptides within the lipid bilayer. Initially, secondary structure preferences and membrane association properties were quantitatively determined for several membrane lipid compositions; they were found to be closely related to those of the natural peptide, indicating that changes in the sequence had little effect on these characteristics of the domain. The tryptophan-substituted peptides became inserted into the membranes' interfacial region, with average tryptophan positions between 7.5 and 11 Å from the bilayer center, in agreement with in-plane orientation of the peptide. Participation of the very-amino terminus of the peptide in the membrane-association process was demonstrated. The results not only revealed the occurrence of association intermediates when the huntingtin 1-17 anchoring sequence became inserted into the membrane but also suggest the formation of aggregates and/or oligomers during membrane association. When inserted, the F11W site was of crucial importance in lipid anchoring and stabilization of the whole peptide, whereas the terminal residues are located close to the membrane surface. The carboxy-terminal tryptophan (F17W), which also constitutes the site of the polyglutamine extension in the natural domain, was found closest to the aqueous environment, accompanied with the highest aqueous quenching constants. These results were used to propose a refined model of lipid interactions of the huntingtin 1-17 domain.

Intramolecular conformational changes optimize protein kinase C signaling

Optimal tuning of enzyme signaling is critical for cellular homeostasis. We use fluorescence resonance energy transfer reporters in live cells to follow conformational transitions that tune the affinity of a multidomain signal transducer, protein kinase C (PKC), for optimal response to second messengers. This enzyme comprises two diacylglycerol sensors, the C1A and C1B domains, that have a sufficiently high intrinsic affinity for ligand so that the enzyme would be in a ligand-engaged, active state if not for mechanisms that mask its domains. We show that both diacylglycerol sensors are exposed in newly synthesized PKC and that conformational transitions following priming phosphorylations mask the domains so that the lower affinity sensor, the C1B domain, is the primary diacylglycerol binder. The conformational rearrangements of PKC serve as a paradigm for how multimodule transducers optimize their dynamic range of signaling.

Single-molecule studies reveal a hidden key step in the activation mechanism of membrane-bound protein kinase C-α

Protein kinase C-α (PKCα) is a member of the conventional family of protein kinase C isoforms (cPKCs) that regulate diverse cellular signaling pathways, share a common activation mechanism, and are linked to multiple pathologies. The cPKC domain structure is modular, consisting of an N-terminal pseudosubstrate peptide, two inhibitory domains (C1A and C1B), a targeting domain (C2), and a kinase domain. Mature, cytoplasmic cPKCs are inactive until they are switched on by a multistep activation reaction that occurs largely on the plasma membrane surface. Often, this activation begins with a cytoplasmic Ca²⁺ signal that triggers C2 domain targeting to the plasma membrane where it binds phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP₂). Subsequently, the appearance of the signaling lipid diacylglycerol (DAG) activates the membrane-bound enzyme by recruiting the inhibitory pseudosubstrate and one or both C1 domains away from the kinase domain. To further investigate this mechanism, this study has utilized single-molecule total internal reflection fluorescence microscopy (TIRFM) to quantitate the binding and lateral diffusion of full-length PKCα and fragments missing specific domain(s) on supported lipid bilayers. Lipid binding events, and events during which additional protein is inserted into the bilayer, were detected by their effects on the equilibrium bound particle density and the two-dimensional diffusion rate. In addition to the previously proposed activation steps, the findings reveal a major, undescribed, kinase-inactive intermediate. On bilayers containing PS or PS and PIP₂, full-length PKCα first docks to the membrane via its C2 domain, and then its C1A domain embeds itself in the bilayer even before DAG appears. The resulting pre-DAG intermediate with membrane-bound C1A and C2 domains is the predominant state of PKCα while it awaits the DAG signal. The newly detected, membrane-embedded C1A domain of this pre-DAG intermediate confers multiple useful features, including enhanced membrane affinity and longer bound state lifetime. The findings also identify the key molecular step in kinase activation: because C1A is already membrane-embedded in the kinase off state, recruitment of C1B to the bilayer by DAG or phorbol ester is the key regulatory event that stabilizes the kinase on state. More broadly, this study illustrates the power of single-molecule methods in elucidating the activation mechanisms and hidden regulatory states of membrane-bound signaling proteins.

Binding of cationic pentapeptides with modified side chain lengths to negatively charged lipid membranes: Complex interplay of electrostatic and hydrophobic interactions

Basic amino acids play a key role in the binding of membrane associated proteins to negatively charged membranes. However, side chains of basic amino acids like lysine do not only provide a positive charge, but also a flexible hydrocarbon spacer that enables hydrophobic interactions. We studied the influence of hydrophobic contributions to the binding by varying the side chain length of pentapeptides with ammonium groups starting with lysine to lysine analogs with shorter side chains, namely omithine (Orn), alpha, gamma-diaminobutyric acid (Dab) and alpha, beta-diaminopropionic acid (Dap). The binding to negatively charged phosphatidylglycerol (PG) membranes was investigated by calorimetry, FT-infrared spectroscopy (FT-IR) and monolayer techniques. The binding was influenced by counteracting and sometimes compensating contributions. The influence of the bound peptides on the lipid phase behavior depends on the length of the peptide side chains. Isothermal titration calorimetry (ITC) experiments showed exothermic and endothermic effects compensating to a different extent as a function of side chain length. The increase in lipid phase transition temperature was more significant for peptides with shorter side chains. FTIR-spectroscopy revealed changes in hydration of the lipid bilayer interface after peptide binding. Using monolayer techniques, the contributions of electrostatic and hydrophobic effects could clearly be observed. Peptides with short side chains induced a pronounced decrease in surface pressure of PG monolayers whereas peptides with additional hydrophobic interactions decreased the surface pressure much less or even lead to an increase, indicating insertion of the hydrophobic part of the side chain into the lipid monolayer.

The C-terminal V5 domain of Protein Kinase Cα is intrinsically disordered, with propensity to associate with a membrane mimetic

The C-terminal V5 domain is one of the most variable domains in Protein Kinase C isoforms (PKCs). V5 confers isoform specificity on its parent enzyme through interactions with isoform-specific adaptor proteins and possibly through specific intra-molecular interactions with other PKC domains. The structural information about V5 domains in solution is sparse. The objective of this work was to determine the conformational preferences of the V5 domain from the α isoform of PKC (V5α) and evaluate its ability to associate with membrane mimetics. We show that V5α and its phosphorylation-mimicking variant, dmV5α, are intrinsically disordered protein domains. Phosphorylation-mimicking mutations do not alter the overall conformation of the polypeptide backbone, as evidenced by the local nature of chemical shift perturbations and the secondary structure propensity scores. However, the population of the “cis-trans” conformer of the Thr⁶³⁸-Pro⁶³⁹-Pro⁶⁴⁰ turn motif, which has been implicated in the down-regulation of PKCα via peptidyl-prolyl isomerase Pin1, increases in dmV5α, along with the conformational flexibility of the region between the turn and hydrophobic motifs. Both wild type and dmV5α associate with micelles made of a zwitterionic detergent, n-dodecylphosphocholine. Upon micelle binding, V5α acquires a higher propensity to form helical structures at the conserved “NFD” motif and the entire C-terminal third of the domain. The ability of V5α to partition into the hydrophobic micellar environment suggests that it may serve as a membrane anchor during the PKC maturation process.

PKC-ε pseudosubstrate and catalytic activity are necessary for membrane delivery during IgG-mediated phagocytosis

In RAW 264.7 cells, PKC-ε regulates FcγR-mediated phagocytosis. BMDM behave similarly; PKC-ε concentrates at phagosomes and internalization are reduced in PKC-ε⁻/⁻ cells. Two questions were asked: what is the role of PKC-ε? and what domains are necessary for PKC-ε concentration? Function was studied using BMDM and frustrated phagocytosis. On IgG surfaces, PKC-ε⁻/⁻ macrophages spread less than WT. Patch-clamping revealed that the spreading defect is a result of the failure of PKC-ε⁻/⁻ macrophages to add membrane. The defect is specific for FcγR ligation and can be reversed by expression of full-length (but not the isolated RD) PKC-ε in PKC-ε⁻/⁻ BMDM. Thus, PKC-ε function in phagocytosis requires translocation to phagosomes and the catalytic domain. The expression of chimeric PKC molecules in RAW cells identified the εPS as necessary for PKC-ε targeting. When placed into (nonlocalizing) PKC-δ, εPS was sufficient for concentration, albeit to a lesser degree than intact PKC-ε. In contrast, translocation of δ(εPSC1B) resembled that of WT PKC-ε. Thus, εPS and εC1B cooperate for optimal phagosome targeting. Finally, cells expressing εK437W were significantly less phagocytic than their PKC-ε-expressing counterparts, blocked at the pseudopod-extension phase. In summary, we have shown that εPS and εC1B are necessary and sufficient for targeting PKC-ε to phagosomes, where its catalytic activity is required for membrane delivery and pseudopod extension.

Kinetics of endophilin N-BAR domain dimerization and membrane interactions

The recruitment to plasma membrane invaginations of the protein endophilin is a temporally regulated step in clathrin-mediated endocytosis. Endophilin is believed to sense or stabilize membrane curvature, which in turn likely depends on the dimeric structure of the protein. The dynamic nature of the membrane association and dimerization of endophilin is thus functionally important and is illuminated herein. Using subunit exchange Förster resonance energy transfer (FRET), we determine dimer dissociation kinetics and find a dimerization equilibrium constant orders of magnitude lower than previously published values. We characterize N-BAR domain membrane association kinetics under conditions where the dimeric species predominates, by stopped flow, observing prominent electrostatic sensitivity of membrane interaction kinetics. Relative to membrane binding, we find that protein monomer/dimer species equilibrate with far slower kinetics. Complementary optical microscopy studies reveal strikingly slow membrane dissociation and an increase of dissociation rate constant for a construct lacking the amphipathic segment helix 0 (H0). We attribute the slow dissociation kinetics to higher-order protein oligomerization on the membrane. We incorporate our findings into a kinetic scheme for endophilin N-BAR membrane binding and find a significant separation of time scales for endophilin membrane binding and subsequent oligomerization. This separation may facilitate the regulation of membrane trafficking phenomena.

Allosteric regulation of protein kinase PKCζ by the N-terminal C1 domain and small compounds to the PIF-pocket

Protein kinases are key mediators of cellular signaling, and therefore, their activities are tightly controlled. AGC kinases are regulated by phosphorylation and by N- and C-terminal regions. Here, we studied the molecular mechanism of inhibition of atypical PKCζ and found that the inhibition by the N-terminal region cannot be explained by a simple pseudosubstrate inhibitory mechanism. Notably, we found that the C1 domain allosterically inhibits PKCζ activity and verified an allosteric communication between the PIF-pocket of atypical PKCs and the binding site of the C1 domain. Finally, we developed low-molecular-weight compounds that bind to the PIF-pocket and allosterically inhibit PKCζ activity. This work establishes a central role for the PIF-pocket on the regulation of PKCζ and allows us to envisage development of drugs targeting the PIF-pocket that can either activate or inhibit AGC kinases.

Protein kinases, from B to C

The PKB (protein kinase B) and PKC (protein kinase C) families display highly related catalytic domains that require a largely conserved series of phosphorylations for the expression of their optimum activities. However, in cells, the dynamics of these modifications are quite distinct. Based on experimental evidence, it is argued that the underlying mechanisms determining these divergent behaviours relate to the very different manner in which their variant regulatory domains interact with their respective catalytic domains. It is concluded that the distinct behaviours of PKB and PKC proteins are defined by the typical ground states of these proteins.

Initial three-dimensional reconstructions of protein kinase C δ from two-dimensional crystals on lipid monolayers

Two-dimensional crystals of protein kinase C delta (PKCdelta) and of its regulatory domain (RDdelta) were grown on lipid monolayers and analyzed by electron microscopy at tilt angles varying from -50 degrees to +55 degrees. Although the crystals exhibit pseudo-3-fold symmetry, analysis of difference phase residuals indicates that there is only one way to align the crystals for merging so the data were processed in plane group P1. Three-dimensional reconstructions generated for several two-dimensional crystals each of PKCdelta and RDdelta show good agreement and are consistent with membrane attachment via a single C1 subdomain, a small surface contact by one or two loops from the C2 domain, and, in intact PKCdelta, a small appendage from the catalytic domain, probably V5. Two-dimensional crystallography with three-dimensional reconstruction should be suitable for examination of additional PKC isozymes and for analysis of the enzymes bound to substrates and other proteins.

Cytoplasmic domain of zebrafish myelin protein zero: adhesive role depends on beta-conformation

Solution spectroscopy studies on the cytoplasmic domain of human myelin protein zero (P0) (hP0-cyt) suggest that H-bonding between beta-strands from apposed molecules is likely responsible for the tight cytoplasmic apposition in compact myelin. As a follow-up to these findings, in the current study we used circular dichroism and x-ray diffraction to analyze the same type of model membranes previously used for hP0-cyt to investigate the molecular mechanism underlying the zebrafish cytoplasmic apposition. This space is significantly narrower in teleosts compared with that in higher vertebrates, and can be accounted for in part by the much shorter cytoplasmic domain in the zebrafish protein (zP0-cyt). Circular dichroism measurements on zP0-cyt showed similar structural characteristics to those of hP0-cyt, i.e., the protein underwent a beta-->alpha structural transition at lipid/protein (L/P) molar ratios >50, and adopted a beta-conformation at lower L/P molar ratios. X-ray diffraction was carried out on lipid vesicle solutions with zP0-cyt before and after dehydration to study the effect of protein on membrane lipid packing. Solution diffraction revealed the electron-density profile of a single membrane bilayer. Diffraction patterns of dried samples suggested a multilamellar structure with the beta-folded P0-cyt located at the intermembrane space. Our findings support the idea that the adhesive role of P0 at the cytoplasmic apposition in compact myelin depends on the cytoplasmic domain of P0 being in the beta-conformation.

Insight into intra- and inter-molecular interactions of PKC: design of specific modulators of kinase function

Protein kinase C (PKC) is a family of kinases that are critical in many cellular events. These enzymes are activated by lipid-derived second messengers, are dependent on binding to negatively charged phospholipids and some members also require calcium to attain full activation. The interaction with lipids and calcium activators is mediated by binding to the regulatory domains C1 and C2. In addition, many protein-protein interactions between PKC and other proteins have been described. These include interactions with adaptor proteins, substrates and cytoskeletal elements. Regulation of the interactions between PKC, small molecules and other proteins is essential for signal transduction to occur. Finally, a number of auto-inhibitory intra-molecular protein-protein interactions have also been identified in PKC. This chapter focuses on mapping the sites for many of these inter- and intra-molecular interactions and how this information may be used to generate selective inhibitors and activators of PKC signaling.

Protein kinase C regulatory domains: the art of decoding many different signals in membranes

Protein kinase C (PKC) is a member of a family of Ser/Thr phosphotransferases that are involved in many cellular signaling pathways. These enzymes possess two regulatory domains, C1 and C2, that are the targets of different second messengers. The purpose of this review is to describe in molecular terms the diverse mechanisms of activation of PKCs in the light of very significant advances made in this field over recent years. The role of some critical amino acid residues concerning activation of the enzymes and their location within known structures of isolated domains will be presented. For example, the recently deduced 3D structures of the C2 domains show that these domains can additionally act as PtdIns(4,5)P(2)-binding or phosphotyrosine-binding modules depending on the isoenzyme. All these capacities to play different roles in the cell wide web of signals underline the notion that we are dealing with a multifunctional family of enzymes which, after 30 years of investigation, we are just beginning to understand.

The ubiquitous phosphoinositides

There are now known to exist seven phosphoinositides all derived through various metabolic routes from the parent lipid phosphatidylinositol. With one additional metabolite, diacylglycerol, these represent a rich resource of bioactive lipids responsible for recruiting protein effectors and marking membrane compartments. The metabolic map of this pathway and the nature of the binding partner interactions are reviewed.

Structural Mechanism for Lipid Activation of the Rac-Specific GAP, β2-Chimaerin

The lipid second messenger diacylglycerol acts by binding to the C1 domains of target proteins, which translocate to cell membranes and are allosterically activated. Here we report the crystal structure at 3.2 A resolution of one such protein, beta2-chimaerin, a GTPase-activating protein for the small GTPase Rac, in its inactive conformation. The structure shows that in the inactive state, the N terminus of beta2-chimaerin protrudes into the active site of the RacGAP domain, sterically blocking Rac binding. The diacylglycerol and phospholipid membrane binding site on the C1 domain is buried by contacts with the four different regions of beta2-chimaerin: the N terminus, SH2 domain, RacGAP domain, and the linker between the SH2 and C1 domains. Phospholipid binding to the C1 domain triggers the cooperative dissociation of these interactions, allowing the N terminus to move out of the active site and thereby activating the enzyme.

The 2004 Biophysical Society-Avanti Award in Lipids address: roles of bilayer structure and elastic properties in peptide localization in membranes

This review details how bilayer structural/elastic properties impact three distinct areas of biological significance. First, the partitioning of melittin into bilayers and melittin-induced bilayer leakage depended strongly on bilayer composition. The incorporation of cholesterol into phosphatidylcholine bilayers decreased melittin-induced leakage from 73 to 3%, and bilayers composed of lipopolysaccharide (LPS), the main lipid on the surface of Gram-negative bacteria, also had low (3%) melittin-induced leakage. Second, transbilayer peptides of different hydrophobic lengths were largely excluded from bilayer microdomains ("rafts") enriched in sphingomyelin (SM) and cholesterol, even when the length of the transbilayer peptide domain matched the hydrocarbon thickness of the raft bilayer. This is likely due to the large area compressibility modulus of SM:cholesterol bilayers. Third, the major water barrier of skin, the extracellular lamellae of the stratum corneum, was found to contain tightly packed asymmetric lipid bilayers with cholesterol located preferentially on one side of the bilayer and a unique skin ceramide containing an unsaturated acyl chain on the opposite side. We argue that, in each of these three areas, key factors are differences in lipid hydrocarbon chain packing for different lipids, particularly the tight hydrocarbon chain packing caused by cholesterol's strong interaction with saturated chains.

Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm

Phosphorylation plays a central role in regulating the activation and signalling lifetime of protein kinases A, B (also known as Akt) and C. These kinases share three conserved phosphorylation motifs: the activation loop segment, the turn motif and the hydrophobic motif. This review focuses on how phosphorylation at each of these sites regulates the maturation, signalling and down-regulation of PKC as a paradigm for how these sites control the function of the ABC kinases.

Surface plasmon resonance spectroscopy in the study of membrane-mediated cell signalling

Peptide-membrane interactions contribute to many important biological processes such as cellular signaling, protein trafficking and ion-channel formation. During receptor-mediated signalling, activated intracellular signalling molecules are often recruited into receptor-induced signaling complexes at the cytoplasmic surface of the cell membrane. Such recruitment can depend upon protein-protein and protein-lipid interactions as well as protein acylation. A wide variety of biophysical techniques have been combined with the use of model membrane systems to study these interactions and have provided important information on the relationship between the structure of these proteins involved in cell signalling and their biological function. More recently, surface plasmon resonance (SPR) spectroscopy has also been applied to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. This article provides an overview of these recent applications, which demonstrate the potential of SPR to enhance our molecular understanding of membrane-mediated cellular signalling.

The use of fluorescent phorbol esters in studies of protein kinase C-membrane interactions

The family of protein kinase C (PKC) isozymes belongs to a growing class of proteins that become active by associating with membranes containing anionic phospholipids, such as phosphatidylserine. Depending on the particular PKC isoform, this process is mediated by Ca(2+)-binding to a C2 domain and interaction of activators such as 1,2-diacyl-sn-glycerol or phorbol esters with tandem C1 domains. This cooperation between the C1 and C2 domains in inducing the association of PKC with lipid membranes provides the energy for a conformational change that consists of the release of a pseudosubstrate sequence from the active site, culminating in activation. Thus, the properties of the interactions of the C1 and C2 domains with membranes, both as isolated domains, and as modules in the full length PKC isoforms, have been the subject of intense scrutiny. Here, we review the findings of studies in which fluorescent phorbol esters have been utilized to probe the properties of the C1 domains of PKC with respect to the interaction with activators, the subsequent interaction with membranes, and the role of the activating conformational change that leads to activation.

Regulation of PKC alpha activity by C1-C2 domain interactions

In this study, the role of interdomain interactions involving the C1 and C2 domains in the mechanism of activation of PKC was investigated. Using an in vitro assay containing only purified recombinant proteins and the phorbol ester, 4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA), but lacking lipids, it was found that PKC alpha bound specifically, and with high affinity, to a alpha C1A-C1B fusion protein of the same isozyme. The alpha C1A-C1B domain also potently activated the isozyme in a phorbol ester- and diacylglycerol-dependent manner. The level of this activity was comparable with that resulting from membrane association induced under maximally activating conditions. Furthermore, it was found that alpha C1A-C1B bound to a peptide containing the C2 domain of PKC alpha. The alpha C1A-C1B domain also activated conventional PKC beta I, -beta II, and -gamma isoforms, but not novel PKC delta or -epsilon. PKC delta and -epsilon were each activated by their own C1 domains, whereas PKC alpha, -beta I, -beta II, or -gamma activities were unaffected by the C1 domain of PKC delta and only slightly activated by that of PKC epsilon. PKC zeta activity was unaffected by its own C1 domain and those of the other PKC isozymes. Based on these findings, it is proposed that the activating conformational change in PKC alpha results from the dissociation of intra-molecular interactions between the alpha C1A-C1B domain and the C2 domain. Furthermore, it is shown that PKC alpha forms dimers via inter-molecular interactions between the C1 and C2 domains of two neighboring molecules. These mechanisms may also apply for the activation of the other conventional and novel PKC isozymes.

Two-dimensional crystal structures of protein kinase C-delta, its regulatory domain, and the enzyme complexed with myelin basic protein

Two-dimensional crystals of protein kinase C (PKC) delta, its regulatory domain (RDdelta), and the enzyme complexed with the substrate myelin basic protein have been grown on lipid monolayers composed of phosphatidylcholine: phosphatidylserine: diolein (45:50:5, molar ratio). Images have been reconstructed to 10-A resolution. The unit cells of all three proteins have cell edges a = b and interedge angle gamma = 60 degrees. RDdelta has an edge length of 33 +/- 1 A, and its reconstruction is donut shaped. The three-dimensional reconstructions from the PKCdelta C1b crystal structure () can be accommodated in this two-dimensional projection. Intact PKCdelta has an edge length of 46 +/- 1 A in the presence or absence of a nonhydrolyzable ATP analog, AMP-PnP. Its reconstruction has a similar donut shape, which can accommodate the C1b domain, but the spacing between donuts is greater than that in RDdelta; some additional structure is visible between the donuts. The complex of PKCdelta and myelin basic protein, with or without AMP-PnP, has an edge length of 43 +/- 1 A and a distinct structure. These results indicate that the C1 domains of RDdelta are tightly packed in the plane of the membrane in the two-dimensional crystals, that there is a single molecule of PKCdelta in the unit cell, and that its interaction with myelin basic protein induces a shift in conformation and/or packing of the enzyme.

Interaction of enkephalin peptides with anionic model membranes

According to the model for passive transport across the membranes, the total flow of permeant molecules is related to the product of the water-membrane partition coefficient and the diffusion coefficient, and to the water-membrane interfacial barrier. The effect of membrane surface charge on the permeability and interaction of analgesic peptide ligands with model membranes was investigated. A mixture of zwitterionic phospholipids with cholesterol was used as a model membrane. The lipid membrane charge density was controlled by the addition of anionic 1-palmitoyl-2-oleoylphosphatidylserine. Two classes of highly potent analgesic peptides were studied, c[D-Pen(2),D-Pen(5)]enkephalin (DPDPE) and biphalin, a dimeric analog of enkephalin. The effect of increased surface charge on the permeability of the zwitterionic DPDPE is a relatively modest decrease, that appears to be due to a diminished partition coefficient. On the other hand the binding of the dicationic biphalin ligands to membranes increases proportionally with increased negative surface charge. This effect translates into a significant reduction of biphalin permeability by reducing the diffusion of the peptide across the bilayer. These experiments show the importance of electrostatic effects on the peptide-membrane interactions and suggest that the negative charge naturally present in cell membranes may hamper the membrane transport of some peptide drugs, especially cationic ones, unless there are cationic transporters present.

Polylysine-induced 2H NMR-observable domains in phosphatidylserine/phosphatidylcholine lipid bilayers

The interaction of three polylysines, Lys(5) (N = 5), Lys(30) (N = 30), and Lys(100) (N = 100), where N is the number of lysine residues per chain, with phosphatidylserine-containing lipid bilayer membranes was investigated using 2H NMR spectroscopy. Lys(30) and Lys(100) added to multilamellar vesicles composed of (70:30) (mol:mol) mixtures of choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) + 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) produced two resolvable 2H NMR spectral components under conditions of low ionic strength and for cases where the global anionic lipid charge was in excess over the global cationic polypeptide charge. The intensities and quadrupolar splittings of the two spectral components were consistent with the existence of polylysine-bound domains enriched in POPS, in coexistence with polylysine-free domains depleted in POPS. Lys(5), however, yielded no 2H NMR resolvable domains. Increasing ionic strength caused domains to become diffuse and eventually dissipate entirely. At physiological salt concentrations, only Lys(100) yielded 2H NMR-resolvable domains. Therefore, under physiological conditions of ionic strength, pH, and anionic lipid bilayer content, and in the absence of other, e.g., hydrophobic, contributions to the binding free energy, the minimum number of lysine residues sufficient to produce spectroscopically resolvable POPS-enriched domains on the 2H NMR millisecond timescale may be fewer than 100, but is certainly greater than 30.

Location and dynamics of basic peptides at the membrane interface: electron paramagnetic resonance spectroscopy of tetramethyl-piperidine-N-oxyl-4-amino-4-carboxylic acid-labeled peptides

The attractive interaction between basic protein domains and membranes containing acidic lipids is critical to the membrane attachment of many proteins involved in cell signaling. In this study, a series of charged model peptides containing lysine, phenylalanine, and the spin-labeled amino acid tetramethyl-piperidine-N-oxyl-4-amino-4-carboxylic acid (TOAC) were synthesized, and electron paramagnetic resonance (EPR) spectroscopy was used to determine their position on the membrane interface and free energy of binding. When membrane-bound, peptides containing only lysine and TOAC assume an equilibrium position within the aqueous double layer at a distance of approximately 5 A from the membrane interface, a result that is consistent with recent computational work. Substitution of two or more lysine residues by phenylalanine dramatically slows the backbone diffusion of these peptides and shifts their equilibrium position by 13-15 A so that the backbone lies several angstroms below the level of the lipid phosphate. These results are consistent with the hypothesis that the position and free energy of basic peptides when bound to membranes are determined by a long-range Coulombic attraction, the hydrophobic effect, and a short-range desolvation force. The differences in binding free energy within this set of charged peptides is not well accounted for by the simple addition of free energies based upon accepted side chain partition free energies, a result that appears to be in part due to differences in membrane localization of these peptides.

Phosphorylation of seminal vesicle protein IV on Ser58 enhances its peroxidase-stimulating activity

In this study we show that SV-IV, a major immunomodulatory, anti-inflammatory, and sperm immunoprotective protein secreted from the rat seminal vesicle epithelium, acts in vitro as a substrate of protein kinase C (PKC) competing efficiently with H1 histone, a very well known PKC substrate. Electrospray mass spectrometry (ES-MS) analysis demonstrated that approximately 10% of the native SV-IV molecules were phosphorylated by PKC and that such a modification involved only a single serine residue (Ser58) out of the 22 occurring in the protein. Interestingly, this modification produced a substantial enhancement (approximately 50%) of the native SV-IV's ability to stimulate the activity of both horseradish peroxidase (POD) and selenium-dependent glutathione peroxidase (GPX), an enzyme that is known to protect the mammalian spermatozoa from oxidative stress and loss of motility in the female genital tract following ejaculation. In contrast, the phosphorylation of SV-IV on Ser58 did not produce any effect on the anti-inflammatory properties of SV-IV, as measured by its ability to inhibit the phospholipase A2.

Effects of bilayer surface charge density on molecular adsorption and transport across liposome bilayers

Second harmonic generation (SHG) was used to study both the adsorption of malachite green (MG), a positively charged organic dye, onto liposomes of different lipid compositions, and the transport kinetics of MG across the liposome bilayer in real time. We found that the dye adsorption increased linearly with the fraction of negatively charged lipids in the bilayer. Similarly, the transport rate constant for crossing the bilayer increased linearly with the fraction of charged lipid in the bilayer.

The C1 and C2 domains of protein kinase C are independent membrane targeting modules, with specificity for phosphatidylserine conferred by the C1 domain

Protein kinase C is specifically activated by binding two membrane lipids: the second messenger, diacylglycerol, and the amino phospholipid, phosphatidylserine. This binding provides the energy to release an autoinhibitory pseudosubstrate from the active site. Interaction with these lipids recruits the enzyme to the membrane by engaging two membrane-targeting modules: the C1 domain (present as a tandem repeat in most protein kinase Cs) and the C2 domain. Here we dissect the contribution of each domain in recruiting protein kinase C betaII to membranes. Binding analyses of recombinant domains reveal that the C2 domain binds anionic lipids in a Ca(2+)-dependent, but diacylglycerol-independent, manner, with little selectivity for phospholipid headgroup beyond the requirement for negative charge. The C1B domain binds membranes in a diacylglycerol/phorbol ester-dependent, but Ca(2+)-independent manner. Like the C2 domain, the C1B domain preferentially binds anionic lipids. However, in striking contrast to the C2 domain, the C1B domain binds phosphatidylserine with an order of magnitude higher affinity than other anionic lipids. This preference for phosphatidylserine is, like that of the full-length protein, stereoselective for sn-1, 2-phosphatidyl-L-serine. Quantitative analysis of binding constants of individual domains and that of full-length protein reveals that the full-length protein binds membranes with lower affinity than expected based on the binding affinity of isolated domains. In addition to entropic and steric considerations, the difference in binding energy may reflect the energy required to expel the pseudosubstrate from the substrate binding cavity. This study establishes that each module is an independent membrane-targeting module with each, independently of the other, containing determinants for membrane recognition. The presence of each of these modules, separately, in a number of other signaling proteins epitomizes the use of these modules as discreet membrane targets.

Macrophage-enriched myristoylated alanine-rich C kinase substrate and its phosphorylation is required for the phorbol ester-stimulated diffusion of beta 2 integrin molecules

An early event of beta(2) integrin activation is the increased diffusion rate of this molecule on the cell surface, thereby providing integrin molecules with a better chance to meet the ligands. The activation of protein kinase C (PKC) stimulates integrin diffusion by releasing the cytoskeletal constraint on integrin molecules. We report here that macrophage-enriched myristoylated alanine-rich C kinase substrate (MacMARCKS), a membrane-associated PKC substrate involved in integrin activation, is required for this PKC-stimulated diffusion of integrin molecules. Using the single-particle tracking technique, we observed that the activation of PKC stimulated an 11-fold increase in the diffusion rate of beta(2) integrins in wild type J774 macrophage cells but not in those expressing mutant MacMARCKS. Further evidence is provided from a MacMARCKS-deficient cell line in which phorbol esters failed to stimulate the diffusion of integrin. Transfection of wild type MacMARCKS into these cells restored the rapid diffusion rate of the beta(2) integrins. The phosphorylation of MacMARCKS is important because transfection of a nonphosphorylatable MacMARCKS mutant or the addition of staurosporine eliminates the rapid diffusion rate of integrin. Furthermore, adding cytochalasin D bypasses the MacMARCKS deficiency and stimulates beta(2) integrin diffusion, suggesting that MacMARCKS's involvement in integrin activation is prior or at the site of cytoskeleton. Therefore, we conclude that MacMARCKS is required for releasing the cytoskeletal constraint on integrin molecules during PKC-mediated integrin activation.

Membrane environment alters the conformational structure of the recombinant human prion protein

The prion protein (PrP) in a living cell is associated with cellular membranes. However, all previous biophysical studies with the recombinant prion protein have been performed in an aqueous solution. To determine the effect of a membrane environment on the conformational structure of PrP, we studied the interaction of the recombinant human prion protein with model lipid membranes. The protein was found to bind to acidic lipid-containing membrane vesicles. This interaction is pH-dependent and becomes particularly strong under acidic conditions. Spectroscopic data show that membrane binding of PrP results in a significant ordering of the N-terminal part of the molecule. The folded C-terminal domain, on the other hand, becomes destabilized upon binding to the membrane surface, especially at low pH. Overall, these results show that the conformational structure and stability of the recombinant human PrP in a membrane environment are substantially different from those of the free protein in solution. These observations have important implications for understanding the mechanism of the conversion between the normal (PrP(C)) and pathogenic (PrP(Sc)) forms of prion protein.

Electrostatic properties of membranes containing acidic lipids and adsorbed basic peptides: theory and experiment

The interaction of heptalysine with vesicles formed from mixtures of the acidic lipid phosphatidylserine (PS) and the zwitterionic lipid phosphatidylcholine (PC) was examined experimentally and theoretically. Three types of experiments showed that smeared charge theories (e.g., Gouy-Chapman-Stern) underestimate the membrane association when the peptide concentration is high. First, the zeta potential of PC/PS vesicles in 100 mM KCl solution increased more rapidly with heptalysine concentration (14.5 mV per decade) than predicted by a smeared charge theory (6.0 mV per decade). Second, changing the net surface charge density of vesicles by the same amount in two distinct ways produced dramatically different effects: the molar partition coefficient decreased 1000-fold when the mole percentage of PS was decreased from 17% to 4%, but decreased only 10-fold when the peptide concentration was increased to 1 microM. Third, high concentrations of basic peptides reversed the charge on PS and PC/PS vesicles. Calculations based on finite difference solutions to the Poisson-Boltzmann equation applied to atomic models of heptalysine and PC/PS membranes provide a molecular explanation for the observations: a peptide adsorbing to the membrane in the presence of other surface-adsorbed peptides senses a local potential more negative than the average potential. The biological implications of these "discreteness-of-charge" effects are discussed.

Interactions controlling the membrane binding of basic protein domains: phenylalanine and the attachment of the myristoylated alanine-rich C-kinase substrate protein to interfaces

Basic residues are known to play a critical role in the attachment of protein domains to membrane interfaces. Many of these domains also contain hydrophobic residues that may alter the binding and the position of the domain on the interface. In the present study, the role of phenylanine in determining the membrane position, dynamics and free energy of a peptide derived from the effector domain of the myristoylated alanine-rich C-kinase substrate (MARCKS) protein was examined. Deuterium NMR in membranes containing phosphatidylcholine (PC) and phosphatidylserine (PS) indicates that this peptide, MARCKS(151-175), partially penetrates the membrane interface when bound and alters the effective charge density on the membrane interface by approximately 2 charges per bound peptide. However, a derivative of this peptide in which the five phenylalanines are replaced by alanine, MARCKS-Ala, does not penetrate the interface when membrane-bound. This result was confirmed by depth measurements by electron paramagnetic resonance spectroscopy on several spin-labeled derivatives of the Phe-less derivative. In contrast to nitroxides on MARCKS(151-175), nitroxides on the derivative lacking Phe do not reside within the bilayer but are in the aqueous phase when the peptide is bound to the membrane. The Phe to Ala substitutions shift the position of the labeled side chains by approximately 10-15 A. The side-chain dynamics of MARCKS-Ala are strongly influenced by membrane charge density and indicate that this peptide is drawn closer to the membrane interface at higher charge densities. As expected, MARCKS-Ala binds more weakly to membranes composed of PS/PC (1:9) than does the native MARCKS peptide; however, each phenylalanine contributes only 0.2 kcal/mol to the binding energy difference, far less than the 1.3 kcal/mol expected for the binding of phenylalanine to the membrane interface. This energetic discrepancy and the differences in membrane position of these peptides can be accounted for by a dehydration energy that is encountered as the peptide approaches the membrane interface. This energy likely includes a Born repulsion acting between the charged peptide and the low dielectric membrane interior. The interplay between the long-range attractive Coulombic force, the short-range repulsive force and the hydrophobic effect controls the position and energetics of protein domains on acidic membrane interfaces.