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Perspectives from Protein-Lipid Interaction of membrane-spanning proteins with peripheral and lipid-anchored membrane proteins: perspectives from protein ± lipid interactions (Review) D. Marsh{*, L. I. HorvaÂth{, M. J. Swamy}, translational diffusion in fluid bilayers, they can be distin- S. Mantripragada} and J. H. Kleinschmidt# guished from the latter in the spin label ESR spectrum. This is possible whenever the lipid rotational mobility differs substantially in the two environments (see figure 1). Then the {Max-Planck-Institut fuÈr biophysikalische Chemie, Abt. stoichiometry and specificity of lipid ± protein interaction can Spektroskopie, 37070 GoÈttingen, Germany be determined by using difference spectroscopyto quantitate the `free’ and protein-interacting lipid populations (Marsh {Institute of Biophysics, Biological Research Centre, 6701 1989). Szeged, Hungary Analysis of the lipid stoichiometry (Nb) and specificity (Kr) from the two-component ESR spectra uses the equation for }School of Chemistry, University of Hyderabad, Hyderabad equilibrium exchange association. The fraction, f, of spin- 5000046, India labelled lipid that is motionally restricted at one of the Nb sites at the intramembranous surface of the protein is given by }Skye Pharma, San Diego, CA 92121, USA (see Marsh 1985): #UniversitaÈt Konstanz, Fachbereich Biologie, 78457 f NbKr l nt Nb Kr 1 1 ˆ ‰ ‡ … ¡ †Š … † Konstanz, Germany where Kr is the average association constant of the spin- Summary labelled lipid relative to the host lipid and nt is the total lipid/ Studies of lipid ± protein interactions in double-reconstituted protein ratio. ESR titrations performed by varying the lipid/ systems involving both integral and peripheral or lipid-anchored protein ratio confirm that a fixed stoichiometry, Nb, is proteins are reviewed. Membranes of dimyristoyl phosphatidyl- maintained independent of nt (Brotherus et al. 1981), and glycerol containing either myelin proteolipid protein or cyto- yield values for both N and the mean relative association chrome c oxidase were studied. The partner peripheral proteins b bound to these membranes were myelin basic protein or constant Kr. In general, Kr&1 for a particular spin-labelled cytochrome c, respectively. In addition, the interactions between lipid (e.g. phosphatidylcholine) relative to its unlabelled the myelin proteolipid protein and avidin that was membrane- parent host lipid. This allows the determination of stoichio- anchored by binding to N-biotinyl phosphatidylethanolamine metries from measurements at a single lipid/protein ratio. were studied in dimyristoyl phosphatidylcholine membranes. It is expected that the lipid stoichiometry at the intramem- Steric exclusion plays a significant role when sizes of the peripheral protein and transmembrane domain of the integral branous perimeter is related directly to the transmembrane protein are comparable. Even so, the effects on avidin-linked structure and degree of oligomerization of the protein (Marsh lipids are different from those induced by myelin basic protein on 1997). Figure 2 compares ESR-determined values of Nb with freely diffusible lipids, both interacting with the myelin proteo- predictions for simple, regular helix packing arrangements. lipid protein. Both the former and the cytochrome c/cytochrome oxidase couple evidence a propagation of lipid perturbation out For a single monomeric helix (phospholamban mutant), and from the intramembrane protein interface that could be a basis for a monomeric 7-helix sandwich (rhodopsin), the predic- for formation of microdomains. tions work well. For proteolipid hexamers (PLP and 16kD), the stoichiometries per monomer are predictably reduced. Keywords: Lipid ± protein interactions, spin labels, electron spin For large, highly polytopic proteins such as cytochrome resonance, myelin proteins, cytochrome oxidase, avidin-biotin. oxidase (or acetylcholine receptor), with more complex and/ or less regular transmembrane arrangements, the simple models do not apply. Further discussion of this topic can be Introduction found in Marsh (1997) and Marsh and Horva th (1998). Electron spin resonance (ESR) with spin-labelled lipids at It is further expected that the selectivity patterns of lipid ± probe amounts has proved to be one of the most valuable protein interaction will reflect in detail the structure and spectroscopic methods for studying lipid ± protein interac- sequence of those sections of transmembrane protein tions in biological membranes (see e.g. Marsh 1985). This is segments that are located in the vicinity of the lipid because the dynamic sensitivity of spin-label ESR is headgroups. This expectation is borne out in practice by optimally matched to the timescale of the rotational motions the results of ESR studies. Figure 3 gives the values of Kr for of the lipid chains in biological membranes, which lies in the representative lipids interacting with a range of different ns range. Even if the exchange rate of the lipids at the membrane proteins. The lipid selectivity patterns differ for the intramembranous surface of an integral protein is as fast as different proteins. Whereas the highest selectivitiesare found for anionic lipids, they are not identical for lipids with the same formal charge, nor does a single lipid display the *To whom correspondence should be addressed. highest selectivity for all proteins. Cytochrome oxidase e-mail: dmarsh@gwdg.de displays its highest selectivity for cardiolipin, a lipid unique Molecular Membrane Biology ISSN 0968-7688 print/ISSN 1464-5203 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/09687680210162419 248 D. Marsh et al. Figure1. Schematic indication of the rotational mobility of the lipid andprotein components in membranes. The grey/ colourscale representsthe rotational correlation time of the different compo- nents.Motional restriction of the spin-labelled lipids in direct contact withthe transmembrane four-helix bundle results in a two-compo- nentESR spectrumfrom which the number of first-shell lipids is quantitated.The transmembrane topology of the myelin proteolipid Figure2. Dependence of the number of first-shell lipids, Nb per proteinis that determined in Weimbs and Stoffel (1992). The polar proteinmonomer, on the actual or predicted number of transmem- loopdeleted in theDM-20 isoform is also indicated. branehelices, na,forthe following integral proteins: M13, phage coat protein;PLB, L37A mutant of phospholamban; PLP, myelin proteolipidprotein; 16 kD, 16-kDa proteolipid from Nephrops; ADP, ADP-ATP carrier;Rho, rhodopsin; Ca, Ca-ATPase; NaK, Na,K- tomitochondria in eucaryotes, whereas rhodopsin displays ATPase;CR, cytochromereductase; AChR, acetylcholinereceptor; littleselectivity for any particular lipid species. This topic is CO, cytochromeoxidase. See Marsh (1997) for references. dealtwith further in Marsh (1995) and Marsh and Horva  th Predictionsfor helical sandwiches or regular polygonal arrange- (1998). ments(solid line) are given, where Da (=1 nm) and dch (=0.48 nm) Figures2 and3 showthat the database on lipid arethe diameters of an a-helixand a lipidchain, respectively. Dotted anddashed lines are corresponding predictions for protein dimers interactionswith single integral membrane protein species andhexamers, respectively. isreasonably extensive. As already implied, this area has beenreviewed several times, one of themost recent being in Marshand Horva th(1998), which also includes consideration ofthe penetration of soluble proteins into membranes. A forbiotin, this system serves as a modelfor covalent lipid considerablebody of work also exists on lipid interactions anchors.Also, because phospholipases must bind specifi- withsingle peripheral proteins. Some of thiswas reviewed in callyto thelipid headgroups, the avidin/ biotinlipid interaction Sankaramand Marsh (1993) and (Marsh (1995). Far fewer maybe used to model certain features of their mode of biophysicalstudies have been undertaken on the mutual interaction. interactionsof integral and peripheral proteins in defined Theway in which the lipid chains anchor the water-soluble reconstitutedsystems. This review is devoted specifically to avidinprotein to membranes was studied by using biotin- thisaspect of protein ± lipidinteractions, including protein- phosphatidylethanolamines(biotin-PE) that are spin-labelled linkedlipid chains. The latter are considered first, in the in their sn-2chain (Swamy and Marsh 1997). These absenceof a secondprotein, by using avidin bound to N- biotinylatedlipids were incorporated in membranes of biotinylphosphatidylethanola mineas amodelsystem. Then, phosphatidylcholine,an inert host lipid that alone does not themyelin basic protein/ myelinproteolipid and cytochrome bindavidin. Figure 4 givesthe lipid chain flexibility profile as a c/cytochromeoxidase couples are treated, followed finally by functionof position, n,ofspinlabelling, and the modification lipid-linkedavidin interacting with the proteolipid protein. ofthis profile by binding avidin. A systematicdecrease in the spectralouter hyperfine splitting, Amax,isfound as the spin- labelis stepped down the chain towards the centre of the Avidin/biotin-lipidmembrane anchoring membrane.This is a measureof the angular amplitude of Phosphatidylethanolaminesthatare N-derivatisedwith biotin motionalfreedom of the lipid chain segments. The char- canspecifically bind the protein avidin (see e.g. Swamy and acteristicflexibility gradient thus registered by the spin- Marsh2001). Because of the extremely high affinity of avidin labelledlipid
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