Solution structure of the pleckstrin homology domain of Drosophila beta-spectrin

β-H-Spectrin is a key component of an apical-medial hub of proteins during cell wedging in tube morphogenesis

Coordinated cell shape changes are a major driver of tissue morphogenesis, with apical constriction of epithelial cells leading to tissue bending. We previously identified that interplay between the apical-medial actomyosin, which drives apical constriction, and the underlying longitudinal microtubule array has a key role during tube budding of salivary glands in the Drosophila embryo. At this microtubule-actomyosin interface, a hub of proteins accumulates, and we have shown before that this hub includes the microtubule-actin crosslinker Shot and the microtubule minus-end-binding protein Patronin. Here, we identify two actin-crosslinkers, β-heavy (H)-Spectrin (also known as Karst) and Filamin (also known as Cheerio), and the multi-PDZ-domain protein Big bang as components of the protein hub. We show that tissue-specific degradation of β-H-Spectrin leads to reduction of apical-medial F-actin, Shot, Patronin and Big bang, as well as concomitant defects in apical constriction, but that residual Patronin is still sufficient to assist microtubule reorganisation. We find that, unlike Patronin and Shot, neither β-H-Spectrin nor Big bang require microtubules for their localisation. β-H-Spectrin is instead recruited via binding to apical-medial phosphoinositides, and overexpression of the C-terminal pleckstrin homology domain-containing region of β-H-Spectrin (β-H-33) displaces endogenous β-H-Spectrin and leads to strong morphogenetic defects. This protein hub therefore requires the synergy and coincidence of membrane- and microtubule-associated components for its assembly and function in sustaining apical constriction during tubulogenesis.

PI(4,5)P2: signaling the plasma membrane

In the almost 70 years since the first hints of its existence, the phosphoinositide, phosphatidyl-D-myo-inositol 4,5-bisphosphate has been found to be central in the biological regulation of plasma membrane (PM) function. Here, we provide an overview of the signaling, transport and structural roles the lipid plays at the cell surface in animal cells. These include being substrate for second messenger generation, direct modulation of receptors, control of membrane traffic, regulation of ion channels and transporters, and modulation of the cytoskeleton and cell polarity. We conclude by re-evaluating PI(4,5)P2's designation as a signaling molecule, instead proposing a cofactor role, enabling PM-selective function for many proteins.

βII spectrin (SPTBN1): biological function and clinical potential in cancer and other diseases

βII spectrin, the most common isoform of non-erythrocyte spectrin, is a cytoskeleton protein present in all nucleated cells. Interestingly, βII spectrin is essential for the development of various organs such as nerve, epithelium, inner ear, liver and heart. The functions of βII spectrin include not only establishing and maintaining the cell structure but also regulating a variety of cellular functions, such as cell apoptosis, cell adhesion, cell spreading and cell cycle regulation. Notably, βII spectrin dysfunction is associated with embryonic lethality and the DNA damage response. More recently, the detection of altered βII spectrin expression in tumors indicated that βII spectrin might be involved in the development and progression of cancer. Its mutations and disorders could result in developmental disabilities and various diseases. The versatile roles of βII spectrin in disease have been examined in an increasing number of studies; nonetheless, the exact mechanisms of βII spectrin are still poorly understood. Thus, we summarize the structural features and biological roles of βII spectrin and discuss its molecular mechanisms and functions in development, homeostasis, regeneration and differentiation. This review highlight the potential effects of βII spectrin dysfunction in cancer and other diseases, outstanding questions for the future investigation of therapeutic targets. The investigation of the regulatory mechanism of βII spectrin signal inactivation and recovery may bring hope for future therapy of related diseases.

Dystrophin and Spectrin, Two Highly Dissimilar Sisters of the Same Family

Dystrophin and Spectrin are two proteins essential for the organization of the cytoskeleton and for the stabilization of membrane cells. The comparison of these two sister proteins, and with the dystrophin homologue utrophin, enables us to emphasise that, despite a similar topology with common subdomains and a common structural basis of a three-helix coiled-coil, they show a large range of dissimilarities in terms of genetics, cell expression and higher level structural organisation. Interactions with cellular partners, including proteins and membrane phospholipids, also show both strikingly similar and very different behaviours. The differences between dystrophin and spectrin are also illustrated by the large variety of pathological anomalies emerging from the dysfunction or the absence of these proteins, showing that they are keystones in their function of providing a scaffold that sustains cell structure.

The Physiological Molecular Shape of Spectrin: A Compact Supercoil Resembling a Chinese Finger Trap

The primary, secondary, and tertiary structures of spectrin are reasonably well defined, but the structural basis for the known dramatic molecular shape change, whereby the molecular length can increase three-fold, is not understood. In this study, we combine previously reported biochemical and high-resolution crystallographic data with structural mass spectroscopy and electron microscopic data to derive a detailed, experimentally-supported quaternary structure of the spectrin heterotetramer. In addition to explaining spectrin’s physiological resting length of ~55-65 nm, our model provides a mechanism by which spectrin is able to undergo a seamless three-fold extension while remaining a linear filament, an experimentally observed property. According to the proposed model, spectrin’s quaternary structure and mechanism of extension is similar to a Chinese Finger Trap: at shorter molecular lengths spectrin is a hollow cylinder that extends by increasing the pitch of each spectrin repeat, which decreases the internal diameter. We validated our model with electron microscopy, which demonstrated that, as predicted, spectrin is hollow at its biological resting length of ~55-65 nm. The model is further supported by zero-length chemical crosslink data indicative of an approximately 90 degree bend between adjacent spectrin repeats. The domain-domain interactions in our model are entirely consistent with those present in the prototypical linear antiparallel heterotetramer as well as recently reported inter-strand chemical crosslinks. The model is consistent with all known physical properties of spectrin, and upon full extension our Chinese Finger Trap Model reduces to the ~180-200 nm molecular model currently in common use.

Spectrins are cytoskeletal and scaffolding proteins ubiquitously expressed in essentially all cell-types. Despite unequivocal evidence for a short physiological length of ~55–65 nm at rest, spectrin is typically represented as an extended ~200 nm molecule that is implied based on crystallographic structures of a number of tandem repeats. Here, we incorporate previously reported biochemical and crystallographic data with structural mass spectroscopy and electron microscopic data to derive a detailed, experimentally-supported quaternary structure of the physiological compact form of spectrin. In addition to explaining spectrin’s physiological resting length (~55–65 nm), our model provides a mechanism by which spectrin can undergo a seamless three-fold extension, which is an experimentally observed property that is responsible for restoration of cell shape after mechanical deformation.

Spectrin and phospholipids - the current picture of their fascinating interplay

The spectrin-based membrane skeleton is crucial for the mechanical stability and resilience of erythrocytes. It mainly contributes to membrane integrity, protein organization and trafficking. Two transmembrane protein macro-complexes that are linked together by spectrin tetramers play a crucial role in attaching the membrane skeleton to the cell membrane, but they are not exclusive. Considerable experimental data have shown that direct interactions between spectrin and membrane lipids are important for cell membrane cohesion. Spectrin is a multidomain, multifunctional protein with several distinctive structural regions, including lipid-binding sites within CH tandem domains, a PH domain, and triple helical segments, which are excellent examples of ligand specificity hidden in a regular repetitive structure, as recently shown for the ankyrin-sensitive lipid-binding domain of beta spectrin. In this review, we summarize the state of knowledge about interactions between spectrin and membrane lipids.

Spectrin: structure, function and disease

Spectrin is a large, cytoskeletal, and heterodimeric protein composed of modular structure of α and β subunits, it typically contains 106 contiguous amino acid sequence motifs called "spectrin repeats". Spectrin is crucial for maintaining the stability and structure of the cell membrane and the shape of a cell. Moreover, it contributes to diverse cell functions such as cell adhesion, cell spreading, and the cell cycle. Mutations of spectrin lead to various human diseases such as hereditary hemolytic anemia, type 5 spinocerebellar ataxia, cancer, as well as others. This review focuses on recent advances in determining the structure and function of spectrin as well as its role in disease.

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Evolution of the spectrin-based membrane skeleton

A group of four proteins - spectrin, ankyrin, 4.1 and adducin - evolved with the metazoa. These membrane-cytoskeletal proteins cross-link actin on the cytoplasmic face of plasma membranes and link a variety of transmembrane proteins to the cytoskeleton. In this paper, the evolution of these proteins is analysed. Genomics indicate that spectrin was the first to appear, since the genome of the choanoflagellate Monosiga brevicolis contains genes for alpha, beta and betaH spectrin. This organism represents a lineage of free-living and colonial protists from which the metazoa are considered to have diverged. This indicates that spectrin emerged in evolution before the animals. Simple animals such as the placozoan Trichoplax adherens also contain recognizable precursors of 4.1, ankyrin and adducin, but these could probably not bind spectrin. Ankyrin and adducin seem to have acquired spectrin-binding activity with the appearance of tissues since they appear to have largely the same domain structure in all eumetazoa. 4.1 was adapted more recently, with the emergence of the vertebrates, to bind spectrin and promote its interaction with actin. A simple hypothesis is that spectrin was prerequisite (but not sufficient) for animal life; that spectrin interaction with ankyrin and adducin was required for evolution of major tissues; and that 4.1 acquired a spectrin-actin binding activity as animal size increased with the appearance of vertebrates. The spectrin/ankyrin/adducin/4.1 complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

The cell adhesion molecule Roughest depends on beta(Heavy)-spectrin during eye morphogenesis in Drosophila

Cell junctions have both structural and morphogenetic roles, and contain complex mixtures of proteins whose interdependencies are still largely unknown. Junctions are also major signaling centers that signify correct integration into a tissue, and modulate cell survival. During Drosophila eye development, the activity of the immunoglobulin cell adhesion molecule Roughest (also known as Irregular chiasm C-roughest protein) mediates interommatidial cell (IOC) reorganization, leading to an apoptotic event that refines the retinal lattice. Roughest and the cadherin-based zonula adherens (ZA) are interdependent and both are modulated by the apical polarity determinant, Crumbs. Here we describe a novel relationship between the Crumbs partner beta(Heavy)-spectrin (beta(H)), the ZA and Roughest. Ectopic expression of the C-terminal segment 33 of beta(H) (betaH33) induces defects in retinal morphogenesis, resulting the preferential loss of IOC. This effect is associated with ZA disruption and Roughest displacement. In addition, loss-of-function karst and roughest mutations interact to cause a synergistic and catastrophic effect on retinal development. Finally, we show that beta(H) coimmunoprecipitates with Roughest and that the distribution of Roughest protein is disrupted in karst mutant tissue. These results suggest that the apical spectrin membrane skeleton helps to coordinate the Cadherin-based ZA with Roughest-based morphogenesis.

Dynamic regulation of CD45 lateral mobility by the spectrin-ankyrin cytoskeleton of T cells

The leukocyte common antigen, CD45, is a critical immune regulator whose activity is modulated by cytoskeletal interactions. Components of the spectrin-ankyrin cytoskeleton have been implicated in the trafficking and signaling of CD45. We have examined the lateral mobility of CD45 in resting and activated T lymphocytes using single-particle tracking and found that the receptor has decreased mobility caused by increased cytoskeletal contacts in activated cells. Experiments with cells that have disrupted betaI spectrin interactions show decreased cytoskeletal contacts in resting cells and attenuation of receptor immobilization in activated cells. Applying two types of population analyses to single-particle tracking trajectories, we find good agreement between the diffusion coefficients obtained using either a mean squared displacement analysis or a hidden Markov model analysis. Hidden Markov model analysis also reveals the rate of association and dissociation of CD45-cytoskeleton contacts, demonstrating the importance of this analysis for measuring cytoskeleton binding events in live cells. Our findings are consistent with a model in which multiple cytoskeletal contacts, including those with spectrin and ankyrin, participate in the regulation of CD45 lateral mobility. These interactions are a major factor in CD45 immobilization in activated cells. Furthermore, cellular activation leads to CD45 immobilization by reduction of the CD45-cytoskeleton dissociation rate. Short peptides that mimic spectrin repeat domains alter the association rate of CD45 to the cytoskeleton and cause an apparent decrease in dissociation rates. We propose a model for CD45-cytoskeleton interactions and conclude that the spectrin-ankyrin-actin network is an essential determinant of immunoreceptor mobility.

Membrane domains based on ankyrin and spectrin associated with cell-cell interactions

Nodes of Ranvier and axon initial segments of myelinated nerves, sites of cell-cell contact in early embryos and epithelial cells, and neuromuscular junctions of skeletal muscle all perform physiological functions that depend on clustering of functionally related but structurally diverse ion transporters and cell adhesion molecules within microdomains of the plasma membrane. These specialized cell surface domains appeared at different times in metazoan evolution, involve a variety of cell types, and are populated by distinct membrane-spanning proteins. Nevertheless, recent work has shown that these domains all share on their cytoplasmic surfaces a membrane skeleton comprised of members of the ankyrin and spectrin families. This review will summarize basic features of ankyrins and spectrins, and will discuss emerging evidence that these proteins are key players in a conserved mechanism responsible for assembly and maintenance of physiologically important domains on the surfaces of diverse cells.

Proteomic tools for the analysis of cytoskeleton proteins

Proteomic tools have become an essential part of the tool kit of the molecular biologist, and provide techniques for detecting homologous sequences, recognizing functional domains, modeling, and analyzing the three-dimensional structure for any given protein sequence. Although a wealth of structural and functional information is available for a large number of members of the various classes of cytoskeletal proteins, many more members remain uncharacterized. These computational tools that are freely and easily accessible to the scientific community provide an excellent starting point to predict the structural and functional properties of such partially or fully uncharacterized protein sequences, and can lead to elegantly designed experiments to probe the hypothesized function. This chapter discusses various proteomic analysis tools with a focus on protein structure and function predictions.

Dynamic interaction between Arf GAP and PH domains of ASAP1 in the regulation of GAP activity

ASAP family Arf GAPs induce the hydrolysis of GTP bound to the Ras superfamily protein Arf1, regulate cell adhesion and migration and have been implicated in carcinogenesis. The ASAP proteins have a core catalytic domain of PH, Arf GAP and Ank repeat domains. The PH domain is necessary for both biological and catalytic functions of ASAP1 and has been proposed to be integrally folded with the Arf GAP domain. Protection studies and analytical ultracentrifugation studies previously reported indicated that the domains are, at least partly, folded together. Here, using NMR spectroscopy and biochemical analysis, we have further tested this hypothesis and characterized the interdomain interaction. A comparison of NMR spectra of three recombinant proteins comprised of either the isolated PH domain of ASAP1, the Arf GAP and ankyrin repeat domain or all three domains indicated that the PH domain did interact with the Arf GAP and Ank repeat domains; however, we found a significant amount of dynamic independence between the PH and Arf GAP domains, consistent with the interactions being transient. In contrast, the Arf GAP and Ank repeat domains form a relatively rigid structure. The PH-Arf GAP domain interaction partially occluded the phosphoinositide binding site in the soluble protein, but binding studies indicated the PIP2 binding site was accessible in ASAP1 bound to a lipid bilayer surface. Phosphoinositide binding altered the conformation of the PH domain, but had little effect on the structure of the Arf GAP domain. Mutations in a loop of the PH domain that contacts the Arf GAP domain affected PIP2 binding and the K(m) and k(cat) for converting Arf1 GTP to Arf1 GDP. Based on these results, we generated a homology model of a composite PH/Arf GAP/Ank repeat domain structure. We propose that the PH domain contributes to Arf GAP activity by either binding to or positioning Arf1 GTP that is simultaneously bound to the Arf GAP domain.

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 beta-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 (PIP(2)) but not phosphatidylinositol 3,4,5-trisphosphate. A K8Q mutation in the lipid binding region of the PH domain eliminated the PIP(2) interaction in vitro, yet the mutant protein retained full biological function in vivo. Reporter gene studies revealed that PIP(2) and the spectrin PH domain codistribute with one another in cells but not with authentic wild type alphabeta-spectrin. Thus, it appears that the PH domain imparts membrane targeting activity through a second mechanism that takes precedence over its PIP(2) binding activity.

Non-canonical Interaction of Phosphoinositides with Pleckstrin Homology Domains of Tiam1 and ArhGAP9

Pleckstrin homology (PH) domains are phosphoinositide (PI)-binding modules that target proteins to membrane surfaces. Here we define a family of PH domain proteins, including Tiam1 and ArhGAP9, that demonstrates specificity for PI(4,5)P(2), as well as for PI(3,4,5)P(3) and PI(3,4)P(2), the products of PI 3-kinase. These PH domain family members utilize a non-canonical phosphoinositide binding pocket related to that employed by beta-spectrin. Crystal structures of the PH domain of ArhGAP9 in complex with the headgroups of Ins(1,3,4)P(3), Ins(1,4,5)P(3), and Ins(1,3,5)P(3) reveal how two adjacent phosphate positions in PI(3,4)P(2), PI(4,5)P(2), and PI(3,4,5)P(3) are accommodated through flipped conformations of the bound phospholipid. We validate the non-canonical site of phosphoinositide interaction by showing that binding pocket mutations, which disrupt phosphoinositide binding in vitro, also disrupt membrane localization of Tiam1 in cells. We posit that the diversity in PI interaction modes displayed by PH domains contributes to their versatility of use in biological systems.

Spectrin functions upstream of ankyrin in a spectrin cytoskeleton assembly pathway

Prevailing models place spectrin downstream of ankyrin in a pathway of assembly and function in polarized cells. We used a transgene rescue strategy in Drosophila melanogaster to test contributions of four specific functional sites in beta spectrin to its assembly and function. (1) Removal of the pleckstrin homology domain blocked polarized spectrin assembly in midgut epithelial cells and was usually lethal. (2) A point mutation in the tetramer formation site, modeled after a hereditary elliptocytosis mutation in human erythrocyte spectrin, had no detectable effect on function. (3) Replacement of repetitive segments 4-11 of beta spectrin with repeats 2-9 of alpha spectrin abolished function but did not prevent polarized assembly. (4) Removal of the putative ankyrin-binding site had an unexpectedly mild phenotype with no detectable effect on spectrin targeting to the plasma membrane. The results suggest an alternate pathway in which spectrin directs ankyrin assembly and in which some important functions of spectrin are independent of ankyrin.

Role of the adaptor proteins Bam32, TAPP1 and TAPP2 in lymphocyte activation

Adaptor proteins play critical roles in lymphocyte activation by mediating intermolecular interactions and assembling signaling complexes at the activated plasma membrane. Bam32/DAPP1 and the related adaptor proteins TAPP1 and TAPP2 were identified by multiple groups about 5 years ago and considerable progress has been made in elucidating the structure, interaction partners and function of these molecules. These cytoplasmic adaptor proteins are recruited to the plasma membrane through interaction of their PH domains with the lipid products of phosphatidylinositol 3-kinases. They share a unique mode of regulation in that they bind with high affinity to phosphatidylinositol-3,4-bisphosphate and their recruitment is enhanced rather than inhibited by the lipid phosphatase SHIP. Two knockout mouse studies and several gain-and-loss of function studies in cell lines have recently been published, demonstrating multiple functions of Bam32 in B cell activation. Bam32 is required for biological responses including B cell antigen receptor (BCR)-induced proliferation and antibody responses to type II T-independent antigens. Bam32 regulates multiple BCR signaling events including activation of the mitogen activated protein kinases ERK and JNK, remodeling of the actin cytoskeleton through the GTPase Rac1 and BCR internalization. Several studies have emerged suggesting that TAPP1 and TAPP2 may play roles in B and T cell activation; however, the biological functions regulated by these molecules remain to be defined. Here we will comprehensively review the available data on the structure and function of Bam32, TAPP1 and TAPP2 and present an integrated working model for Bam32 function in B cell activation and a general model for distinct effector pathways of PI 3-kinases.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

Cytoskeletal regulation: rich in lipids

Phosphorylated derivatives of the phospholipid phosphatidylinositol, or phosphoinositides, are implicated in many aspects of cell function. Binding of phosphoinositides that are localized within cell membranes to soluble protein ligands allows spatially selective regulation at the cytoplasm-membrane interface. Recently, studies that relate phosphoinositide production to membrane domains are converging with those that show effects of these lipids on the assembly of cellular actin, and are therefore linking membrane and cytoskeletal structures in new ways.

The C-terminal domain ofDrosophilaβHeavy-spectrin exhibits autonomous membrane association and modulates membrane area

Current models of cell polarity invoke asymmetric cues that reorganize the secretory apparatus to induce polarized protein delivery. An important step in this process is the stabilization of the protein composition in each polarized membrane domain. The spectrin-based membrane skeleton is thought to contribute to such stabilization by increasing the half-life of many proteins at the cell surface. Genetic evidence is consistent with a negative role for Drosophila βHeavy-spectrin in endocytosis, but the inhibitory mechanism has not been elucidated. Here, we investigated the membrane binding properties of the C-terminal nonrepetitive domain of βHeavy-spectrin through its in vivo expression in transgenic flies. We found that this region is a membrane-association domain that requires a pleckstrin homology domain for full activity, and we showed for the first time that robust membrane binding by such a C-terminal domain requires additional contributions outside the pleckstrin homology. In addition, we showed that expression of the βHeavy-spectrin C-terminal domain has a potent effect on epithelial morphogenesis. This effect is associated with its ability to induce an expansion in plasma membrane surface area. The membrane expansions adopt a very specific bi-membrane structure that sequesters both the C-terminal domain and the endocytic protein dynamin. Our data provide supporting evidence for the inhibition of endocytosis by βHeavy-spectrin, and suggest that the C-terminal domain mediates this effect through interaction with the endocytic machinery. Spectrin may be an active partner in the stabilization of polarized membrane domains.

Membrane targeting by pleckstrin homology domains

Pleckstrin homology (PH) domains are small modular domains that occur once, or occasionally several times, in a large variety of signalling proteins. In a number of instances, PH domains act to target their host protein to the cytosolic face of cellular membranes through an ability to associate with phosphoinositides. In this review, we discuss recent advances in our understanding of PH domain function. In particular we describe the structural aspects of how PH domains have evolved to bind various phosphoinositides, how PH domains regulate phosphoinositide-mediated association to plasma and internals membranes, and finally raise the issue of PH domains in protein:protein interactions and the allosteric regulation of their host protein.

Engineering the phosphoinositide-binding profile of a class I pleckstrin homology domain

Pleckstrin homology (PH) domains are protein modules that bind with varying degrees of affinity and specificity membrane phosphoinositides. Previously we have shown that although the PH domains of the Ras GTPase-activating proteins GAP1m and GAP1IP4BP are 63% identical at the amino acid level they possess distinct phosphoinositide-binding profiles. The GAP1m PH domain binds phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3), whereas the domain from GAP1IP4BP binds PtdIns(3,4,5)P3 and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) equally well. These phosphoinositide specificities are translated into distinct subcellular localizations. GAP1m is cytosolic and undergoes a rapid PtdIns(3,4,5)P3-dependent association with the plasma membrane following growth factor stimulation. In contrast, GAP1IP4BP is constitutively associated, in a PtdIns(4,5)P2-dependent manner, with the plasma membrane (Cozier, G. E., Lockyer, P. J., Reynolds, J. S., Kupzig, S., Bottomley, J. R., Millard, T., Banting, G., and Cullen, P. J. (2000) J. Biol. Chem. 275, 28261-28268). In the present study, we have used molecular modeling to identify residues in the GAP1IP4BP PH domain predicted to be required for high affinity binding to PtdIns(4,5)P2. This has allowed the isolation of a mutant, GAP1IP4BP-(K591T), which while retaining high affinity for PtdIns(3,4,5)P3 has a 6-fold reduction in its affinity for PtdIns(4,5)P2. Importantly, GAP1IP4BP-(K591T) is predominantly localized to the cytosol and undergoes a PtdIns(3,4,5)P3-dependent association with the plasma membrane following growth factor stimulation. We have therefore engineered the phosphoinositide-binding profile of the GAP1IP4BP PH domain, thereby emphasizing that subtle changes in PH domain structure can have a pronounced effect on phosphoinositide binding and the subcellular localization of GAP1IP4BP.

Occurrence of lipid receptors inferred from brain and erythrocyte spectrins binding NaOH-extracted and protease-treated neuronal and erythrocyte membranes

It was previously shown in model systems that brain spectrin binds membrane phospholipids. In the present study, we analysed binding of isolated brain spectrin and red blood cell spectrin to red blood or neuronal membranes which had been treated as follows: (1). extracted with low ionic-strength solution, (2). the above membranes extracted with 0.1 M NaOH, and (3). membranes treated as above, followed by protease treatment and re-extraction with 0.1 M NaOH. It was found that isolated, NaOH-extracted, protease-treated neuronal and red blood cell membranes bind brain and red blood cell spectrin with moderate affinities similar to those obtained in model phospholipid membrane-spectrin interaction experiments. Moreover, this binding was competitively inhibited by liposomes prepared from membrane lipids. The presented results indicate the occurrence of receptor sites for spectrins that are extraction- and protease-resistant, therefore most probably of lipidic nature, in native membranes.

Towards a complete atomic structure of spectrin family proteins

The spectrin family of proteins represents a discrete group of cytoskeletal proteins comprising principally alpha-actinin, spectrin, dystrophin, and homologues and isoforms. They all share three main structural and functional motifs, namely, the spectrin repeat, EF-hands, and a CH domain-containing actin-binding domain. These proteins are variously involved in organisation of the actin cytoskeleton, membrane cytoskeleton architecture, cell adhesion, and contractile apparatus. The highly modular nature of these molecules has been a hindrance to the determination of their complete structures due to the inherent flexibility imparted on the proteins, but has also been an asset, inasmuch as the individual modules were of a size amenable to structural analysis by both crystallographic and NMR approaches. Representative structures of all the major domains shared by spectrin family proteins have now been solved at atomic resolution, including in some cases multiple domains from several family members. High-resolution structures, coupled with lower resolution methods to determine the overall molecular shape of these proteins, allow us for the first time to build complete atomic structures of the spectrin family of proteins.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Structure of a PH domain from the C. elegans muscle protein UNC-89 suggests a novel function

BACKGROUND

Pleckstrin homology (PH) domains constitute a structurally conserved family present in many signaling and regulatory proteins. PH domains have been shown to bind to phospholipids, and many function in membrane targeting. They generally have a strong electrostatic polarization and interact with negatively charged phospholipids via the positive pole. On the basis of electrostatic modeling, however, we have previously identified a class of PH domains with a predominantly negative charge and predicted that these domains recognize other targets. Here, we report the first experimental structure of such a PH domain.

RESULTS

The structure of the PH domain from Caenorhabditis elegans muscle protein UNC-89 has been determined by heteronuclear NMR. The domain adopts the classic PH fold, but has an unusual closed conformation of the "inositol binding loops. This creates a small opening to a deep hydrophobic pocket lined with negative charges on one side, and provides a molecular explanation for the lack of association with inositol-1,4,5-triphosphate. As predicted, the PH domain of UNC-89 has a strongly negative overall electrostatic potential. Modeling the Dbl homology (DH)-linked PH domains from the C. elegans genome shows that a large proportion of these modules are negatively charged.

CONCLUSIONS

We present the first structure of a PH domain with a strong negative overall electrostatic potential. The presence of a deep pocket lined with negative charges suggests that the domain binds to ligands other than acidic phospholipids. The abundance of this class of PH domain in the C. elegans genome suggests a prominent role in mediating protein-protein interactions.

Molecular modelling and site-directed mutagenesis of the inositol 1,3,4,5-tetrakisphosphate-binding pleckstrin homology domain from the Ras GTPase-activating protein GAP1IP4BP

GAP1(IP4BP) is a Ras GTPase-activating protein (GAP) that in vitro is regulated by the cytosolic second messenger inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P(4)]. We have studied Ins(1,3,4,5)P(4) binding to GAP1(IP4BP), and shown that the inositol phosphate specificity and binding affinity are similar to Ins(1,3,4,5)P(4) binding to Bruton's tyrosine kinase (Btk), evidence which suggests a similar mechanism for Ins(1,3,4,5)P(4) binding. The crystal structure of the Btk pleckstrin homology (PH) domain in complex with Ins(1,3,4,5)P(4) has shown that the binding site is located in a partially buried pocket between the beta 1/beta 2- and beta 3/beta 4-loops. Many of the residues involved in the binding are conserved in GAP1(IP4BP). Therefore we generated a model of the PH domain of GAP1(IP4BP) in complex with Ins(1,3,4,5)P(4) based on the Btk-Ins(1,3,4,5)P(4) complex crystal structure. This model had the typical PH domain fold, with the proposed binding site modelling well on the Btk structure. The model has been verified by site-directed mutagenesis of various residues in and around the proposed binding site. These mutations have markedly reduced affinity for Ins(1,3,4,5)P(4), indicating a specific and tight fit for the substrate. The model can also be used to explain the specificity of inositol phosphate binding.

SH3 domains and drug design: ligands, structure, and biological function

The ligand binding preferences, structural features, and biological function of SH3 (Src homology 3) domains are discussed. SH3 domains bind "core" Pro-rich peptide ligands (7-9 amino acids in length) in a polyproline II helical conformation in a highly conserved aromatic rich patch on the protein surface (approximately 390 A2). The ligands can interact with the protein in one of two orientations, depending on the position (N- vs C-terminal) of ligand residues binding to the SH3 selectivity pocket. Core SH3 ligands are characterized by relatively weak interactions (KD = 5-100 microM) that show little binding selectivity within SH3 families. Higher affinity, more selective contiguous ligands require additional flanking residues that bind to less conserved portions of the SH3 surface, with corresponding increase in ligand size and complexity. In contrast to peptide ligands, protein ligands of SH3 domains can exploit multiple discontiguous interactions to enhance affinity and selectivity. A protein-SH3 interaction that utilizes unique interactions may permit the design of small high affinity SH3 ligands. At present, the extended nature of the binding site and homologous nature of the core binding region among SH3 domains present key challenges for structure-based drug design.

Signaling and subcellular targeting by membrane-binding domains

Protein kinase C homology-1 and -2, FYVE, and pleckstrin homology domains are ubiquitous in eukaryotic signal transduction and membrane-trafficking proteins. These domains regulate subcellular localization and protein function by binding to lipid ligands embedded in cell membranes. Structural and biochemical analysis of these domains has shown that their molecular mechanisms of membrane binding depend on a combination of specific and nonspecific interactions with membrane lipids. In vivo studies of green fluorescent protein fusions have highlighted the key roles of these domains in regulating protein localization to plasma and internal membranes in cells.

Classification of protein sequences by homology modeling and quantitative analysis of electrostatic similarity

Protein electrostatics plays a key role in ligand binding and protein-protein interactions. Therefore, similarities or dissimilarities in electrostatic potentials can be used as indicators of similarities or dissimilarities in protein function. We here describe a method to compare the electrostatic properties within protein families objectively and quantitatively. Three-dimensional structures are built from database sequences by comparative modeling. Molecular potentials are then computed for these with a continuum solvation model by finite difference solution of the Poisson-Boltzmann equation or analytically as a multipole expansion that permits rapid comparison of very large datasets. This approach is applied to 104 members of the Pleckstrin homology (PH) domain family. The deviation of the potentials of the homology models from those of the corresponding experimental structures is comparable to the variation of the potential in an ensemble of structures from nuclear magnetic resonance data or between snapshots from a molecular dynamics simulation. For this dataset, the results for analysis of the full electrostatic potential and the analysis using only monopole and dipole terms are very similar. The electrostatic properties of the PH domains are generally conserved despite the extreme sequence divergence in this family. Notable exceptions from this conservation are seen for PH domains linked to a Db1 homology (DH) domain and in proteins with internal PH domain repeats.

Modular PH and C2 domains in membrane attachment and other functions

The pleckstrin homology and C2 domains are modular protein structures involved in mediating intermolecular interactions. Although they represent distinct domains, there are several parallels regarding their function and type of interactions in which they participate. Both domains are stable structural entities that incorporate variable regions which, in different proteins, can be adapted to perform a specific function through binding to membrane phospholipids or specific protein ligands. A number of recent examples illustrate the function of some of these domains in regulated membrane attachment, with an important role in many cellular signalling pathways.

Phospholipid-binding protein domains

Research into cellular mechanisms for signal transduction is currently one of the most exciting and rapidly advancing fields of biological study. It has been known for some time that numerous intracellular signals are transmitted by specific protein-protein interactions, as exemplified by those involving the Src homology domains. However, after some controversy, it has recently been widely accepted that specific protein-phospholipid interactions also play key roles in many signal transduction pathways. In this review, landmark discoveries and recent advances describing protein domains known to associate with phospholipids are discussed. Particular emphasis is placed on the interactions of proteins with phospholipids acting as second messengers in signalling pathways. For this purpose, the pleckstrin homology (PH) domain is highlighted, since studies of this domain provided some of the earliest, detailed data about protein-phospholipid interactions occurring downstream of growth factor-mediated receptor stimulation. Moreover, studies of PH domains have given insight into the mechanisms of certain diseases, revealed a number of intriguing functional variations on a common structural theme and recently culminated in providing the missing links in erstwhile mysteries of phosphoinositide-dependent signal transduction pathways. Finally, a short discussion is devoted to the developing field of protein-phospholipid interactions that influence cytoskeletal organisation.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Pleckstrin homology domains: a common fold with diverse functions

Pleckstrin homology (PH) motifs are approximately 100 amino-acid residues long and have been identified in nearly 100 different eukaryotic proteins, many of which participate in cell signaling and cytoskeletal regulation. Despite minimal sequence homology, the three-dimensional structures are remarkably conserved. This review gives an overview of the PH domain architecture and examines the best-studied examples in an attempt to understand their function.

The solution structure and dynamics of the pleckstrin homology domain of G protein-coupled receptor kinase 2 (beta-adrenergic receptor kinase 1). A binding partner of Gbetagamma subunits

The solution structure of an extended pleckstrin homology (PH) domain from the beta-adrenergic receptor kinase is obtained by high resolution NMR. The structure establishes that the beta-adrenergic receptor kinase extended PH domain has the same fold and topology as other PH domains, and there are several unique features, most notably an extended C-terminal alpha-helix that behaves as a molten helix, and a surface charge polarity that is extensively modified by positive residues in the extended alpha-helix and the C terminus. These observations complement biochemical evidence that the C-terminal portion of this PH domain participates in protein-protein interactions with Gbetagamma subunits. This suggests that the C-terminal segment of the PH domain may function to mediate protein-protein interactions with the targets of PH domains.

Functional diversity of PH domains: an exhaustive modelling study

BACKGROUND

Pleckstrin homology (PH) domains are found in many proteins involved in signal transduction or cytoskeletal organization. The general function for the domain is still unclear; phospholipid binding of some PH domains and a strong electrostatic polarization in the experimental structures suggest a role in localization on membranes. We have analyzed the electrostatic properties and the spatial amino acid distribution from homology models of the entire PH domain family.

RESULTS

Despite the sequence divergence, the quality of the models is sufficient for our study. Most PH domains have an electrostatic polarization similar to the experimental structures. but roughly half of the PH domains linked to a Dbl homology domain have very different electrostatic properties. We also found a striking electrostatic complementarity in some internal PH domain repeats. The analysis of the spatial distribution of amino acids identified residues in the phospholipid-binding site of the spectrin and dynamin PH domains as specific for these domains.

CONCLUSIONS

The mostly conserved electrostatic polarization supports a general function in binding to phospholipid membranes. But the presence of PH domains with opposite polarity suggests that ligands and functions have diverged during evolution. We also demonstrate homology modelling as a general sequence analysis tool that can yield significantly more information than conventional analysis.

The solution structure of the pleckstrin homology domain of mouse Son-of-sevenless 1 (mSos1)

The solution structure of the pleckstrin homology (PH) domain of mouse Son-of-sevenless 1 (mSos1), a guanine nucleotide exchange factor for Ras, was determined by multidimensional NMR spectroscopy. The structure of the mSos1 PH domain involves the fundamental PH fold, consisting of seven beta-strands and one alpha-helix at the C terminus, as determined for the PH domains of other proteins. By contrast, the mSos1 PH domain showed two major characteristic features. First, the N-terminal region, whose amino acid sequence is highly conserved among Sos proteins, was found to form an alpha-helix, which interacts with the beta-sheet structure of the fundamental PH fold. Second, there is a long unstructured loop between beta3 and beta4. Furthermore, the mSos1 PH domain was found to bind phosphatidylinositol-4,5-bisphosphate by a centrifugation assay. The addition of inositol-1,4,5-trisphosphate to the mSos1 PH domain induced backbone amide chemical shift changes mainly in the beta1/beta2 loop and the N- and C-terminal parts of the long beta3/beta4 loop. This inositol-1,4,5-trisphosphate-binding mode of the mSos1 PH domain is somewhat similar to those of the PH domains of pleckstrin and phospholipase Cdelta1, and is clearly different from those of other PH domains.

Automated NOESY interpretation with ambiguous distance restraints: the refined NMR solution structure of the pleckstrin homology domain from beta-spectrin

We have used a novel, largely automated, calculation method to refine the NMR solution structure of the pleckstrin homology domain of beta-spectrin. The method is called ARIA for Ambiguous Restraints for Iterative Assignment. The starting point for ARIA is an almost complete assignment of the proton chemical shifts, and a list of partially assigned NOEs, mostly sequential and secondary structure NOEs. The restraint list is then augmented by automatically interpreting peak lists generated by automated peak-picking. The central task of ARIA is the assignment of ambiguous NOEs during the structure calculation using a combination of ambiguous distance restraints and an iterative assignment strategy. In addition, ARIA calibrates ambiguous NOEs to derive distance restraints, merges overlapping data sets to remove duplicate information, and uses empirical rules to identify erroneous peaks. While the distance restraints for the structure calculations were exclusively extracted from homonuclear 2D experiments, ARIA is especially suited for the analysis of multidimensional spectra. Applied to the pleckstrin homology domain, ARIA generated structures of good quality, and of sufficiently high accuracy to solve the X-ray crystal structure of the same domain by molecular replacement. The comparison of the free NMR solution structure to the X-ray structure, which is complexed to D-myo-inositol-1,4,5-triphosphate, shows that the ligand primarily induces a disorder-order transition in the binding loops, which are disordered in the NMR ensemble but well ordered in the crystal. The structural core of the protein is unaffected, as evidenced by a backbone root-mean-square difference between the average NMR coordinates and the X-ray crystal structure for the secondary structure elements of less than 0.6 A.

Can enzymatic activity, or otherwise, be inferred from structural studies of annexin III?

Annexin III, a putative inositol (1,2)-phosphohydrolase, was co-crystallized with inositol 2-phosphate, the inhibitor of the reaction, and its structure was solved to 1.95 A resolution. No enzyme active site was observed in the structure. Assays for enzymatic activity were also negative. Search for annexin III-inositol phosphate interactions using the BIAcoreTM system revealed an affinity for inositol cyclic (1,2)-phosphate, suggesting annexin III may sequester the molecule in the cell. The BIAcoreTM system used with different phospholipids showed that annexin III displays specificity for phosphatidylethanolamine, but not for phosphatidylinositols. Interestingly, a molecule of ethanolamine was found bound to the protein in the crystal structure. Coupled with the fact that this is a particularly abundant phospholipid in granules specific to neutrophils, cells where annexin III is highly expressed, our finding could be pointing to a physiological role of annexin III.

A structural tree for proteins containing 3beta-corners

A structural tree for beta-proteins with predominantly orthogonal beta-sheet packing has been constructed. The 3beta-corner, a structural motif that recurs in proteins of this class, is taken as a root structure of the tree. The 3beta-corner can be represented as a triple-stranded beta-sheet folded on to itself so that its two beta-beta-hairpins are packed approximately orthogonally in different layers and the central strand bends by approximately 90 degrees in a right-handed direction when passing from one layer to the other. The larger protein structures are obtained by stepwise addition of beta-strands to the root 3beta-corner taking into account a restricted set of rules inferred from known principles of protein structure. The protein structures that can be obtained in this way are grouped into one structural class and those found in branches of the structural tree into subclasses.

The main-chain dynamics of the dynamin pleckstrin homology (PH) domain in solution: analysis of 15N relaxation with monomer/dimer equilibration

The backbone dynamics of the pleckstrin homology (PH) domain from dynamin were studied by 15N NMR relaxation (R1 and R2) and steady state heteronuclear 15N [1H] nuclear Overhauser effect measurements at 500 and 600 MHz, at protein concentrations of 1.7 mM and 300 microM, and by molecular dynamics (MD) simulations. The analysis was performed using the model-free approach. The method was extended in order to account for observed partial (equilibrium) dimerization of the protein at NMR concentrations. A model is developed that takes into account both rapid monomer-dimer exchange and anisotropy of the over-all rotation of the dimer. The data show complex dynamics of the dynamin PH domain. Internal motions in elements of the secondary structure are restricted, as inferred from the high value of the order parameter (S2 approximately 0.9) and from the local correlation time < 100 ps. Of the four extended loop regions that are disordered in the NMR-derived solution structure of the protein, loops beta 1/beta 2 and beta 5/beta 6 are involved in a large-amplitude (S2 down to 0.2 to 0.3) subnanosecond to nanosecond time-scale motion. Reorientation of the loops beta 3/beta 4 and beta 6/beta 7, in contrast, is restricted, characterized by the values of order parameter S2 approximately 0.9 more typical of the protein core. These loops, however, are involved in much slower processes of motion resulting in a conformational exchange on a microsecond to submillisecond time scale. The motions of the terminal regions (residues 1 to 10, 122 to 125) are practically unrestricted (S2 down to 0.05, characteristic times in nanosecond time scale), suggesting that these parts of the sequence do not participate in the protein fold. The analysis shows a larger sensitivity of the 15N relaxation data to protein microdynamic parameters (S2, tau loc) when protein molecular mass (tau c) increases. The use of negative values of the steady state 15N[1H] NOEs as an indicator of the residues not belonging to the folded structure is suggested. The amplitudes of local motion observed in the MD simulation are in a good-agreement with the NMR data for the amide NH groups located in the protein core.

Structural basis for IL-4 receptor phosphopeptide recognition by the IRS-1 PTB domain

We present the NMR structure of the PTB domain of insulin receptor substrate-1 (IRS-1) complexed to a tyrosine-phosphorylated peptide derived from the IL-4 receptor. Despite the lack of sequence homology and different binding specificity, the overall fold of the protein is similar to that of the Shc PTB domain and closely resembles that of PH domains. However, the PTB domain of IRS-1 is smaller than that of Shc (110 versus 170 residues) and binds to phosphopeptides in a distinct manner. We explain the phosphopeptide binding specificity based on the structure of the complex and results of site-directed mutagenesis experiments.

Spectrin: on the path from structure to function

New structural analyses of the spectrin family of actin cross-linking proteins are providing molecular explanations for both the interchain binding between the alpha and beta chains of spectrin and the intermolecular associations between spectrin and other proteins. Additionally, the analyses bring into focus a conformation which may explain aspects of spectrin's interaction with lipids.