Review
The role of electrostatics in protein–membrane interactions

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Abstract

Many experimental, structural and computational studies have established the importance of nonspecific electrostatics as a driving force for peripheral membrane association. Here we focus on this component of protein/membrane interactions by using examples ranging from phosphoinositide signaling to retroviral assembly. We stress the utility of the collaboration of experiment and theory in identifying and quantifying the role of electrostatics not only in contributing to membrane association, but also in affecting subcellular targeting, in the control of membrane binding, and in the organization of proteins and lipids at membrane surfaces.

Section snippets

The importance of electrostatics in the membrane association of peripheral proteins

The reversible binding of proteins to membrane surfaces is critical to many biological processes and is often accomplished through lipid-interacting protein domains. The membrane association of many peripheral membrane proteins has been shown to be mediated, at least in part, by electrostatic interactions [26], [58], [82]. It is well established that a number of proteins, such as Src, K-Ras, and MARCKS require the nonspecific electrostatic interaction between a cluster of basic residues on the

Computational approaches

The finite difference Poisson–Boltzmann (FDPB) method [31], [54], [104] has been widely applied to describe the electrostatic properties of proteins, nucleic acids, and membranes [8], [10], [82]. In a series of experimental/theoretical studies over the past decade, we and others have used this method to describe the binding of charged peptides and proteins to membrane surfaces. This work demonstrated that the FDPB method is remarkably accurate in treating electrostatic properties associated

The balance of electrostatic and non-polar contributions

Secreted Phospholipases A2 (sPL2's) have long served as a paradigm for interfacial association. sPLA2's are generally recruited to membrane surfaces through a combination of nonspecific electrostatic and hydrophobic interactions, the balance of which can be quite different for different sPLA2 groups [44]. Indeed, many peripheral proteins contain hydrophobic residues on or near their basic surfaces that have been shown to penetrate the membrane interface. The experimental interfacial

The effect of lipid composition on protein/membrane electrostatics

“Common” phospholipids, such as the zwitterionic, electrically neutral lipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE), and the monovalent acidic lipid phosphatidylserine (PS), are relatively abundant in cellular membranes [17], [37]. For example, PC and PE constitute ∼ 60% and PS constitutes ∼ 25% of the phospholipid in the inner leaflet of the plasma membrane [125]. Hence, the cytosolic surface of the plasma membrane carries an appreciable negative charge due to the large

Membrane aggregation

Fig. 5A shows an atypical but very interesting example of an sPLA2, the human group IIA sPLA2 (hGIIA sPLA2), whose binding is largely driven by electrostatic interactions [108]. Its net charge is + 17e and its electrostatic potential profile is almost completely positive. Vesicle binding measurements were confounded by vesicle aggregation when the acidic lipid composition was higher than 25 mol% [22]. FDPB calculations predict that there are two roughly equivalent minimum electrostatic free

Discreteness of charge

A number of proteins that contain basic clusters are known to exist at high concentrations in localized regions of the plasma membrane [77]. For example, MARCKS is concentrated at nascent phagosomes in macrophages [4] and HIV-1 Gag self-assembles into lateral domains on the cytoplasmic surface of the plasma membrane of infected cells before viral budding [18], [101]. Experiments and FDPB calculations with atomic models of peptides and membranes describe the electrostatic properties of membranes

Phosphoinositides

Compared to other phospholipids, phosphoinositides are present in cells at very low levels [70], [90], [100]: Phosphatidylinositol comprises ∼ 4% of cellular membrane phospholipid, and its phosphorylated derivatives, the phosphoinositides, together comprise ∼ 1%. The phosphoinositides are designated according to the positions that are phosphorylated; for example phosphatidylinositol 4,5-bisphosphate, or PI(4,5)P2, is the phosphoinositide obtained by phosphorylation of the inositol ring of

Electrostatic mechanisms for regulating protein/membrane association

Electrostatics is not only a driving force for peripheral membrane association but also a major contributor to ligand-specific mediated events, e.g. the interaction of a FYVE domain with a mixed membrane containing PC, PS and PI(3)P. Table 2, while not exhaustive, lists some of the ways in which membrane association is regulated by nonspecific electrostatic effects. The mechanisms in this table will be described throughout the rest of this review as examples of the strikingly diverse

Protein phosphorylation

The membrane partitioning of a basic peptide corresponding to the N-terminal portion of the non-receptor tyrosine kinase Src, which is crucial for the function of the intact protein, was examined experimentally and computationally with the FDPB method [85]. The calculations accurately predict how the membrane binding is altered as a function of (1) the mole percent acidic lipid in the membrane, (2) the ionic strength of the solution, (3) the number of basic residues in the peptide, and (4) the

Partner and effector association/dissociation

The basic patch on Gβγ depicted in Fig. 1A is largely occluded when the heterodimer is bound to the Gα subunit in the transducin heterotrimer during the resting state of a cell [67]. Activation of a G protein-coupled receptor signaling pathway and GTP-induced dissociation of Gα from Gβγ, uncovers this basic feature, which is adjacent to the site of prenylation on the Gγ subunit [109]. Likely, the basic patch serves mainly to orient Gβγ at the membrane surface for productive effector

Protonation of interfacial glutamates and histidines

The desolvation of a protein upon membrane association (Fig. 2) can change the electrostatic character of the protein just as explicitly as phosphorylation. The effect is dependent on the close apposition of protein and membrane so that the dielectric and/or electrostatic environment of the protein is significantly altered. Desolvation is a surface or interfacial phenomenon that always opposes binding so that there must always be an attractive membrane binding motif on the protein to provide

Ligand-induced electrostatic switches

The term “switch” was originally coined in regard to C2 domains to describe how calcium binding increases attractive electrostatic interactions with acidic phospholipids [98]. The meaning of this “calcium/electrostatic switch” was expanded to include C2 domains from cytosolic phospholipase A2 [86] and 5-lipoxygenase [65], which have a large negative potential surrounding a cluster of hydrophobic residues. As suggested in Fig. 7A and B for the C2 domain from 8R-lipoxygenase, calcium ions

FYVE domains: a combination of nonspecific and specific components of electrostatic control

FYVE domains contain a basic motif responsible for ligand binding and a small hydrophobic motif adjacent to the ligand binding site [35], [80], [81]. Most FYVE domains bind only PI(3)P [19], [43], [93], which is predominantly found in endosomal compartments. Hence, many FYVE domain-containing proteins function in the endocytic pathway. FYVE domains provide a nice example of how the collaboration of experiment and theory has been able to delineate the contribution of nonspecific electrostatic

Lateral co-localization at membrane surfaces

PI(4,5)P2 is the source of two second messengers in the cell, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) [12], and is crucial for a wide variety of cellular functions [73], [79]. PI(4,5)P2 has been shown to be distributed non-uniformly in the plasma membranes so that the existence of distinct pools have been suggested to help organize its functionality. Nonspecific electrostatic sequestration mediated by membrane-associated MARCKS is one mechanism by which not only

Lateral sequestration of protein domains due to nonspecific electrostatics

Similar to Fig. 6, the electrostatic profile of a membrane surface containing PI(4,5)P2, monovalent acidic lipids and adsorbed basic sequences is expected to be quite complex and may produce driving forces for many types of lateral interactions. As an example, the panels in Fig. 8 illustrate how the electrostatic profiles of PLCδ PH domains change upon binding the headgroup of PI(4,5)P2 [107]. The δ1 PH domain becomes overall negatively charged, while the δ3 PH domain remains highly positively

Retroviral assembly: oligomerization

Retroviruses are enveloped viruses that cause a wide range of diseases in humans and animals [49], [116]. Newly assembled viruses acquire their lipid coats by budding through the plasma membrane of host cells. Gag directs the assembly of new virions and is targeted to the inner leaflet of the plasma membrane by its matrix domain (MA) [126]. It is unclear how Gag is targeted to the host cell plasma membrane during viral assembly, but many studies implicate specific motifs in MA (see e.g., [29],

Multi-component signaling complexes at the plasma membrane surface

Fig. 10A schematically depicts an interesting system in which a number of phenomena based on nonspecific electrostatic interactions stressed throughout this review are manifested. Binding of EGF to the exterior of the receptor (light and dark green, respectively) produces dimerization, which leads to trans-autophosphorylation (not shown). Ligand binding produces a rapid increase in the free intracellular Ca2+ level [69]. Model peptide studies in the McLaughlin and Smith labs indicate that the

Conclusions

There is a significant body of experimental work that points to a central role of electrostatics in mediating the interactions of proteins with membrane surfaces. Calculations on atomistic models of proteins and membrane using implicit solvent models provide a unifying description of the structural and energetic origins of these interactions. The results of these calculations yield a wide range of quantities in agreement with experiment. More importantly, they form the basis of a general

Acknowledgments

D.M. is deeply grateful for collaborations and important discussions with Barry Honig, Wonhwa Cho, Stuart McLaughlin and Volker Vogt. D.M. acknowledges the NIH (grants GM66147 and AI54167), NSF (grant MCB0212362 and grant MCB030028 for advanced computational resources at the Pittsburgh Supercomputing Center), and the Alfred P. Sloan Foundation for supporting this work.

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