Heterogeneous Rieske proteins in the cytochrome b6f complex of Synechocystis PCC6803?

The completely sequenced genome of the cyanobacterium Synechocystis PCC6803 contains three open reading frames, petC1, petC2, and petC3, encoding putative Rieske iron-sulfur proteins. After heterologous overexpression, all three gene products have been characterized and shown to be Rieske proteins as typified by sequence analysis and EPR spectroscopy. Two of the overproduced proteins contained already incorporated iron-sulfur clusters, whereas the third one formed unstable aggregates, in which the FeS cluster had to be reconstituted after refolding of the denatured protein. Although EPR spectroscopy showed typical FeS signals for all Rieske proteins, an unusual low midpoint potential was revealed for PetC3 by EPR redox titration. Detailed characterization of Synechocystis membranes indicated that all three Rieske proteins are expressed under physiological conditions. Both for PetC1 and PetC3 the association with the thylakoid membrane was shown, and both could be identified, although in different amounts, in the isolated cytochrome b(6)f complex. The considerably lower redox potential determined for PetC3 indicates heterogeneous cytochrome b(6)f complexes in Synechocystis and suggests still to be established alternative electron transport routes.     

Comparison of the cytochrome bc1 complex with the anticipated structure of the cytochrome b6f complex: Le plus ça change le plus c'est la même chose

Structural alignment of the integral cytochrome b6-SU IV subunits with the solved structure of the mitochondrial bc1 complex shows a pronounced asymmetry. There is a much higher homology on the p-side of the membrane, suggesting a similarity in the mechanisms of intramembrane and interfacial electron and proton transfer on the p-side, but not necessarily on the n-side. Structural differences between the bc1 and b6f complexes appear to be larger the farther the domain or subunit is removed from the membrane core, with extreme differences between cytochromes c1 and f. A special role for the dimer may involve electron sharing between the two hemes b(p), which is indicated as a probable event by calculations of relative rate constants for intramonomer heme b(p) --> heme b(n), or intermonomer heme b(p) --> heme b(p) electron transfer. The long-standing observation of flash-induced oxidation of only approximately 0.5 of the chemical content of cyt f may be partly a consequence of the statistical population of ISP bound to cytfon the dimer. It is proposed that the p-side domain of cyt f is positioned with its long axis parallel to the membrane surface in order to: (i) allow its large and small domains to carry out the functions of cyt c1 and suVIII, respectively, of the bc1 complex, and (ii) provide maximum dielectric continuity with the membrane. (iii) This position would also allow the internal water chain ("proton wire") of cyt f to serve as the p-side exit port for an intramembrane H+ transfer chain that would deprotonate the semiquinol located in the myxothiazol/MOA-stilbene pocket near heme b(p). A hypothesis is presented for the identity of the amino acid residues in this chain.     

The complex of cytochrome c and cytochrome c peroxidase: the end of the road?

Cytochrome c (Cc) and cytochrome c peroxidase (CcP) form a physiological complex in the inter-membrane space of yeast mitochondria, where CcP reduces hydrogen peroxide to water using the electrons provided by ferrous Cc. The Cc-CcP system has been a popular choice of study of interprotein biological electron transfer (ET) and in understanding dynamics within a protein-protein complex. In this review we have charted seven decades of research beginning with the discovery of CcP and leading to the latest functional and structural work, which has clarified the mechanism of the intermolecular ET, addressed the putative functional role of a low-affinity binding site, and identified lowly-populated intermediates on the energy landscape of complex formation. Despite the remarkable attention bestowed on this complex, a number of outstanding issues remain to be settled on the way to a complete understanding of Cc-CcP interaction.     

Structure of a protein–detergent complex: the balance between detergent cohesion and binding

Despite the major interest in membrane proteins at functional, genomic, and therapeutic levels, their biochemical and structural study remains challenging, as they require, among other things, solubilization in detergent micelles. The complexity of this task derives from the dependence of membrane protein structure on their anisotropic environment, influenced by a delicate balance between many different physicochemical properties. To study such properties in a small protein–detergent complex, we used fluorescence measurements and molecular dynamics (MD) simulations on the transmembrane part of glycophorin A (GpAtm) solubilized in micelles of dihexanoylphosphatidylcholine (DHPC) detergent. Fluorescence measurements show that DHPC has limited ability to solubilize the peptide, while MD provides a possible molecular explanation for this. We observe that the detergent molecules are balanced between two different types of interactions: cohesive interactions between detergent molecules that hold the micelle together, and adhesive interactions with the peptide. While the cohesive interactions are detergent mediated, the adhesion to the peptide depends on the specific interactions between the hydrophobic parts of the detergent and the topography of the peptide dictated by the amino acids. The balance between these two parameters results in a certain frustration of the system and rather slow equilibration. These observations suggest how molecular properties of detergents could influence membrane protein stabilization and solubilization.     

Interaction of monogalactosyldiacylglycerol with cytochrome b₆f complex in surface films

The interaction of monogalactosyldiacylglycerol (MGDG) with cytochrome b(6)f complex (cyt b(6)f), a major component of the photosynthetic apparatus, was studied in Langmuir monolayers during compression/expansion cycling and at constant surface pressure mode. The surface pressure/area isotherms of the mixed films were analyzed in terms of surface compressional modulus and two-dimensional virial equation of state. The morphology and the surface potential of the monolayers were monitored by Brewster angle microscopy and vibrating plate sensor respectively. Our results suggested that there is a specific interaction between MGDG and cyt b(6)f which resulted in depletion of lipid molecules from the interface. The current work sheds light on the still unclear question how b(6)f complex gets in touch with the major compound of the thylakoid membranes, the non-charged lipid MGDG. The interaction occured even at very low sub-nanomolar concentration of the complex. This effect most probably could be attributed to hydrogen bonding between the galactose headgroup of the lipid and the protein moiety of cyt b(6)f.     

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

The fast (up to 1?s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.     

Co-evolution of cytochrome c 6 and plastocyanin, mobile proteins transferring electrons from cytochrome b 6f to photosystem I

 Cytochrome c ₆ and plastocyanin are soluble metalloproteins that act as mobile carriers transferring electrons between the two membrane-embedded photosynthetic complexes cytochrome b ₆ f and photosystem I (PSI). First, an account of recent data on structural and functional features of these two membrane complexes is presented. Afterwards, attention is focused on the mobile heme and copper proteins – and, in particular, on the structural factors that allow recognition and confer molecular specificity and control the rates of electron transfer from and to the membrane complexes. The interesting question of why plastocyanin has been chosen over the ancient heme protein is discussed to place emphasis on the evolutionary aspects. In fact, cytochrome c ₆ and plastocyanin are presented herein as an excellent case study of biological evolution, which is not only convergent (two different structures but the same physiological function), but also parallel (two proteins adapting themselves to vary accordingly to each other within the same organism). Received: 4 July 1996 / Accepted: 16 September 1996     

Structure–function studies on the complex iron–sulfur flavoprotein glutamate synthase: the key enzyme of ammonia assimilation

Glutamate synthases are complex iron–sulfur flavoproteins that participate in the essential ammonia assimilation pathway in microorganisms and plants. The recent determination of the 3-dimensional structures of the α subunit of the NADPH-dependent glutamate synthase form and of the ferredoxin-dependent enzyme of Synechocystis sp. PCC 6803 provides a framework for the interpretation of the functional properties of these enzymes, and highlights protein segments most likely involved in control and coordination of the partial catalytic activities of glutamate synthases, which take place at sites distant from each other in space. In this review, we focus on the current knowledge on structure–function relationships in glutamate synthases, and we discuss open questions on the mechanisms of control of the enzyme reaction and of electron transfer among the enzyme flavin cofactors and iron–sulfur clusters.     

Structure of the UreD–UreF–UreG–UreE complex in Helicobacter pylori: a model study

The molecular details of the protein complex formed by UreD, UreF, UreG, and UreE, accessory proteins for urease activation in the carcinogenic bacterium Helicobacter pylori, have been elucidated using computational modeling. The calculated structure of the complex supports the hypothesis of UreF acting as a GTPase activation protein that facilitates GTP hydrolysis by UreG during urease maturation, and provides a rationale for the design of new drugs against infections by ureolytic bacterial pathogens.     

Light-induced absorption spectra of the D1–D2–cytochrome b 559 complex of Photosystem II: Effect of methyl viologen concentration

The light-induced difference absorption spectra associated to the photo-accumulation of reduced pheophytin a were studied in the isolated D1–D2–Cyt b559 complex in the presence of variable methyl viologen concentrations and different illumination conditions under anaerobiosis. Depending on the methyl viologen/reaction centre ratio, the relative intensities of the spectral bands at 681.5±0.5, 667.0±0.5 and 542.5±0.5 nm were modified. The reduced pheophytin a located at the D1-branch of the complex absorbs at 681.7±0.5 nm, and at least two additional pigment species contribute to the Q_y band of the difference absorption spectra with maxima at 667.0±0.5 and 680.5±0.5 nm. We propose the additional species correspond to a peripheral chlorophyll a and the pheophytin a located at the D2-branch of the complex, respectively. The blue absorbing chlorophyll at 667 nm is susceptible to chemical redox changes with a midpoint reduction potential of +470 mV. The Q_x absorption bands of both pheophytins localised at the D2- and D1-branch of the D1–D2–Cyt b559 complex were at 540.7±0.5 and 542.9±0.5, respectively. The results indicated that the two pheophytin molecules can be photoreduced in the D1–D2–Cyt b559 complex in certain experimental conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of detergents DDM and β-OG on the singlet excited state lifetime of the chlorophyll a in cytochrome b_6f complex from spinach chloroplasts

The singlet excited state lifetime of the chlorophyll a (Chl a) in cytochrome b6f (Cyt b6f) complex was reported to be shorter than that of free Chl a in methanol, but the value was different for Cyt b6f com-plexes from different sources (~200 and ~600 ps are the two measured results). The present study demonstrated that the singlet excited state lifetime is associated with the detergents n-dodecyl-β-D- maltoside (DDM) and n-octyl-β-D-glucopyranoside (β-OG), but has nothing to do with the different sources of Cyt b6f complexes. Compared with the Cyt b6f dissolved in β-OG, the Cyt b6f in DDM had a lower fluorescence yield, a lower photodegradation rate of Chl a, and a shorter lifetime of Chl a excited state. In short, the singlet excited state lifetime, ~200 ps, of the Chl a in Cyt b6f complex in DDM is closer to the true in vivo.     

The Rieske Fe/S protein of the cytochrome b6/f complex in chloroplasts: missing link in the evolution of protein transport pathways in chloroplasts?

The Rieske Fe/S protein, a nuclear-encoded subunit of the cytochrome b(6)/f complex in chloroplasts, is retarded in the stromal space after import into the chloroplast and only slowly translocated further into the thylakoid membrane system. As shown by the sensitivity to nigericin and to specific competitor proteins, thylakoid transport takes place by the DeltapH-dependent TAT pathway. The Rieske protein is an untypical TAT substrate, however. It is only the second integral membrane protein shown to utilize this pathway, and it is the first authentic substrate without a cleavable signal peptide. Transport is instead mediated by the NH(2)-terminal membrane anchor, which lacks, however, the twin-arginine motif indicative of DeltapH/TAT-dependent transport signals. Furthermore, transport is affected by sodium azide as well as by competitor proteins for the Sec pathway in chloroplasts, demonstrating for the first time some cross-talk of the two pathways. This might take place in the stroma where the Rieske protein accumulates after import in several complexes of high molecular mass, among which the cpn60 complex is the most prominent. These untypical features suggest that the Rieske protein represents an intermediate or early state in the evolution of the thylakoidal protein transport pathways.     

How does heme axial ligand deletion affect the structure and the function of cytochrome b(562)?

We have recently generated a new mutant of cytochrome b(562) (cytb(562)) in which Met7, one of the axial heme ligands, is replaced by Ala (M7A cytb(562)). The M7A cytb(562) can bind heme and the UV-visible absorption spectrum is of a typical high-spin ferric heme. To investigate the effect of the lack of Met7 ligation on the structural integrity of cytb(562), thermal transition analyses of M7A cytb(562) were conducted. From the thermodynamic parameters obtained, it is concluded that the folding of M7A cytb(562) is comparable to the apoprotein despite the presence of heme. On the other hand, exogenous ligands such as cyanide and azide ions are readily bound to the heme iron, indicating that the axial coordination site is available for substrate binding. The peroxidase activity of this mutant is thus examined to evaluate new enzymatic function at this site and M7A cytb(562) was found to catalyze an oxidation reaction of aromatic substrates with hydrogen peroxide. These observations demonstrate that the Met7/His102 bis-ligation to the heme iron is crucial for the stable folding of cytb(562), whereas the functional conversion of cytb(562) is successfully achieved by the loose folding together with the open coordination site.     

Structure and ecological function of the soil microbiome affecting plant–soil feedbacks in the presence of a soil-borne pathogen

Interactions between plants and soil microbes are important for plant growth and resistance. Through plant–soil-feedbacks, growth of a plant is influenced by the previous plant that was growing in the same soil. We performed a plant–soil feedback study with 37 grass, forb and legume species, to condition the soil and then tested the effects of plant-induced changes in soil microbiomes on the growth of the commercially important cut-flower Chrysanthemum in presence and absence of a pathogen. We analysed the fungal and bacterial communities in these soils using next-generation sequencing and examined their relationship with plant growth in inoculated soils with or without the root pathogen, Pythium ultimum. We show that a large part of the soil microbiome is plant species-specific while a smaller part is conserved at the plant family level. We further identified clusters of plant species creating plant growth promoting microbiomes that suppress concomitantly plant pathogens. Especially soil inocula with higher relative abundances of arbuscular mycorrhizal fungi caused positive effects on the Chrysanthemum growth when exposed to the pathogen. We conclude that plants differ greatly in how they influence the soil microbiome and that plant growth and protection against pathogens is associated with a complex soil microbial community.     

The binding interface of cytochrome c and cytochrome c₁ in the bc₁ complex: rationalizing the role of key residues

The interaction of cytochrome c with ubiquinol-cytochrome c oxidoreductase (bc₁ complex) has been studied for >30 years, yet many aspects remain unclear or controversial. We report the first molecular dynamic simulations of the cyt c-bc₁ complex interaction. Contrary to the results of crystallographic studies, our results show that there are multiple dynamic hydrogen bonds and salt bridges in the cyt c-c₁ interface. These include most of the basic cyt c residues previously implicated in chemical modification studies. We suggest that the static nature of x-ray structures can obscure the quantitative significance of electrostatic interactions between highly mobile residues. This provides a clear resolution of the discrepancy between the structural data and functional studies. It also suggests a general need to consider dynamic interactions of charged residues in protein-protein interfaces. In addition, a novel structural change in cyt c is reported, involving residues 21-25, which may be responsible for cyt c destabilization upon binding. We also propose a mechanism of interaction between cyt c₁ monomers responsible for limiting the binding of cyt c to only one molecule per bc₁ dimer by altering the affinity of the cytochrome c binding site on the second cyt c₁ monomer.     

Is the Redox State of the ci Heme of the Cytochrome b6f Complex Dependent on the Occupation and Structure of the Qi Site and Vice Versa?

Oxidoreductases of the cytochrome bc₁/b₆f family transfer electrons from a liposoluble quinol to a soluble acceptor protein and contribute to the formation of a transmembrane electrochemical potential. The crystal structure of cyt b₆f has revealed the presence in the Q_i site of an atypical c-type heme, heme c_i. Surprisingly, the protein does not provide any axial ligand to the iron of this heme, and its surrounding structure suggests it can be accessed by exogenous ligand. In this work we describe a mutagenesis approach aimed at characterizing the c_i heme and its interaction with the Q_i site environment. We engineered a mutant of Chlamydomonas reinhardtii in which Phe⁴⁰ from subunit IV was substituted by a tyrosine. This results in a dramatic slowing down of the reoxidation of the b hemes under single flash excitation, suggesting hindered accessibility of the heme to its quinone substrate. This modified accessibility likely originates from the ligation of the heme iron by the phenol(ate) side chain introduced by the mutation. Indeed, it also results in a marked downshift of the c_i heme midpoint potential (from +100 mV to −200 mV at pH 7). Yet the overall turnover rate of the mutant cytochrome b₆f complex under continuous illumination was found similar to the wild type one, both in vitro and in vivo. We propose that, in the mutant, a change in the ligation state of the heme upon its reduction could act as a redox switch that would control the accessibility of the substrate to the heme and trigger the catalysis.The cytochrome b₆f complex of oxygenic photosynthesis is the integral membrane protein, the quinol:plastocyanin oxido-reductase activity of which allows the linear electron flow between the two photosystems (PSI and PSII).⁴ The turnover of the cytochrome b₆f complex depends on the steady state of its redox partners, the liposoluble plastoquinol (PQH₂ reduced and protonated plastoquinone PQ) formed by the PSII, and the hydrosoluble plastocyanin oxidized by the PSI. In the Q_o site, exposed to the lumenal side, the quinol is oxidized, and this oxidation is coupled to the release of two protons into the lumen. The two electrons provided by the quinol are transferred along two bifurcated pathways, the high and low potential chains. The high potential chain involves two lumenal redox partners, the membrane-anchored flexible Rieske [2Fe-2S] cluster and the cytochrome f, which ultimately interacts with the soluble plastocyanin. In the low potential chain, electrons are transferred to the stroma via the low and high potential b hemes (b_L and b_H) of the transmembrane b₆ subunit. Two turnovers of the cyt b₆f complex lead to the reduction of the low potential chain, thereby allowing the reduction of a quinone molecule in the stromal Q_i pocket. This mechanism, which recycles reducing equivalents, is referred to as the “Q cycle,” initially described by Mitchell (1) and modified later by Crofts et al. and others (2, 3).Although this quinol:cytochrome oxidoreductase activity is involved in both the respiratory and photosynthetic electron transfer chains, recent x-ray data (4–6) have evidenced major structural differences between the b₆f complex and its mitochondrial counterpart the bc₁ complex (for reviews see Refs. 7–10). Indeed an additional heme localized in close contact with heme b_H stands as another putative electron carrier as proposed earlier by Lavergne (11). Since it was brought to light by the x-ray studies, knowledge of the basic properties of this heme, named c_i in reference to the Q_i site (5) or c_n in reference to its proximity to the negatively charged side of the membrane (4), has significantly improved. The proteins involved in the assembly machinery of the heme have been identified in Chlamydomonas reinhardtii and Arabidopsis thaliana (12, 13). Consistent with the structure, according to which the only axial ligand could be a water molecule interacting with the proponiate chain of the b_H heme, the spectroscopic properties of this heme are those of a high spin heme (14, 15). Evidences for a high spin heme covalently bound to the cytochrome b subunit were also found in Heliobacterium modesticaldum and Heliobacillus mobilis (16).In the b₆f complex from the oxygenic photosynthetic chain, EPR (15) and structural data (17) have shown that NQNO (2-n-nony l-4-hydroxyquinoline N-oxide), an inhibitor of the Q_i pocket (18, 19), can act as an axial ligand to c_i. This ligation is accompanied by a significant change in the redox properties of c_i, because, in the presence of NQNO, at least two titrations waves were observed (13, 14), one with a midpoint potential (E_m) similar to that observed in the absence of NQNO and the other with a midpoint potential downshifted by ∼250 mV. This, together with the widespread range of redox potential found for heme c_i (11, 14, 15, 20), points to a structural plasticity of the c_i ligand network.This plasticity may arise from the unusual coordination properties of the heme c_i. As a matter of fact, the x-ray models of the complex from C. reinhardtii and Mastigocladus laminosus evidenced a water or hydroxyl molecule as a fifth ligand. The sixth position of coordination is directed toward the Q_i pocket and appears as free. Nevertheless, the side chain of Phe⁴⁰ of subunit IV protrudes above the heme plane, leaving little space for any axial ligand to the heme c_i. Besides, modeling a quinone analogue in the electron density found in the Q_i pocket of structures obtained in presence of Tridecyl- Stigmatellin or NQNO implies a steric clash with the native position of the Phe⁴⁰ aromatic ring.⁵ The Phe⁴⁰ residue of subunit IV therefore stands as a key residue for the plasticity of the site, making it an ideal mutagenesis target when attempting to alter the possible interactions between c_i and the quinone or quinol (4, 5) (Fig. 1). Here we present the consequences of the substitution of Phe⁴⁰ by a tyrosine on the properties and function of the c_i heme.Open in a separate windowFIGURE 1.A view of the Q_i site from the dimer interface. The coordinates are from the Protein Data Bank 1Q90 model. The Van der Waal''s surface of the peptide chains was drawn with Pymol. Phe⁴⁰ is in van der Waal''s contact with the plane of the c_i, heme.     

Structure–function analysis of the barley genome: the gene-rich region of chromosome 2HL

A major gene-rich region on the end of the long arm of Triticeae group 2 chromosomes exhibits high recombination frequencies, making it an attractive region for positional cloning. Traits known to be controlled by this region include chasmogamy/cleistogamy, frost tolerance at flowering, grain yield, head architecture, and resistance to Fusarium head blight and rusts. To assist these cloning efforts, we constructed detailed genetic maps of barley chromosome 2H, including 61 polymerase chain reaction markers. Colinearity with rice occurred in eight distinct blocks, including five blocks in the terminal gene-rich region. Alignment of rice sequences from the junctions of colinear chromosome segments provided no evidence for the involvement of long (>2.5 kb) inverted repeats in generating inversions. However, reuse of some junction sequences in two or three separate evolutionary breakage/fusion events was implicated, suggesting the presence of fragile sites. Sequencing across 91 gene fragments totaling 107 kb from four barley genotypes revealed the highest single nucleotide substitution and insertion–deletion polymorphism levels in the terminal regions of the chromosome arms. The maps will assist in the isolation of genes from the chromosome 2L gene-rich region in barley and wheat by providing markers and accelerating the identification of the corresponding points in the rice genome sequence. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structure–function correlations of pulmonary surfactant protein SP-B and the saposin-like family of proteins

Pulmonary surfactant is a lipid-protein complex secreted by the respiratory epithelium of mammalian lungs, which plays an essential role in stabilising the alveolar surface and so reducing the work of breathing. The surfactant protein SP-B is part of this complex, and is strictly required for the assembly of pulmonary surfactant and its extracellular development to form stable surface-active films at the air–liquid alveolar interface, making the lack of SP-B incompatible with life. In spite of its physiological importance, a model for the structure and the mechanism of action of SP-B is still needed. The sequence of SP-B is homologous to that of the saposin-like family of proteins, which are membrane-interacting polypeptides with apparently diverging activities, from the co-lipase action of saposins to facilitate the degradation of sphingolipids in the lysosomes to the cytolytic actions of some antibiotic proteins, such as NK-lysin and granulysin or the amoebapore of Entamoeba histolytica. Numerous studies on the interactions of these proteins with membranes have still not explained how a similar sequence and a potentially related fold can sustain such apparently different activities. In the present review, we have summarised the most relevant features of the structure, lipid-protein and protein–protein interactions of SP-B and the saposin-like family of proteins, as a basis to propose an integrated model and a common mechanistic framework of the apparent functional versatility of the saposin fold.