Pores barrel-stave

Although several studies on linear amphipathic a-helical AMPs showed the formation of transmembrane pores probably through the barrel-stave mechanism, only a few of them could be confirmed. Among them are pardaxin, alamethicin, and the helix a5 of the 5-endotoxin [87]. 

Figure 18.1 Models for different modes of peptide-lipid interaction of membrane-active peptides. The peptide remains unstructured in solution and acquires an amphipathic structure in the presence of a membrane. The hydrophobic face of the amphipathic peptide binds to the membrane, as represented by the grayscale. At low concentration, the peptide lies on the surface. At higher peptide concentrations the membrane becomes disrupted, either by the formation of transmembrane pores or by destabilization via the "carpet mechanism." In the "barrel-stave pore" the pore consists of peptides alone, whereas in the "toroidal wormhole pore" negatively charged lipids also line the pore, counteracting the electrostatic repulsion between the positively charged peptides. The peptide may also act as a detergent and break up the membrane to form small aggregates. Peptides can also induce inverted micelle structures in the membrane.

Figure 18.3 The orientation of a peptide in the membrane can be described by the tilt angle x and the azimuthal angle p. x is the angle between the bilayer normal (n) and the peptide long axis, p describes a rotation around the peptide long axis and must be defined with respect to a reference group as indicated by the white circle. In liquid-crystalline bilayers, peptides can usually also rotate around the membrane normal (shown by the dashed arrow). Three characteristic peptide orientations are shown in the S-state the peptide lies flat on the membrane surface with charged amino acids facing the water in the T-state the peptide is inserted with an oblique tilt into the membrane, possibly in a dimeric state (shown as a second peptide in white) and in the inserted l-state the peptide has a transmembrane orientation. In this state, the peptide may self-assemble into pores (shown here as a barrel-stave pore together with additional white peptides).

Virtual vertical cutting of this unimolecular barrel gives the barrel-stave , horizontal cutting the barrel-hoop , horizontal and vertical cutting the barrel-rosette motif (Fig. 11.2). More complex motifs that include modification of the lipid bilayer are summarized as micellar pores. 

The barrel-stave architecture is a classic for both biological and synthetic ion channels and pores (Fig. 11.3). Whereas barrel-hoop motifs have received considerable attention, barrel-rosette ion channels and pores are just beginning to emerge. The more complex micellar (or toroidal ) ion channels and pores are, on the one hand, different from detergents because the micellar defects introduced into the lipid bilayer are only transient. Micellar pores differ, on the other hand, from membrane-spanning (i.e. transmembrane ) barrel-stave pores because (a) they disturb the bilayer suprastructure and (b) they always remain at the membrane-water interface. A representative synthetic barrel-stave pore is shown in Fig. 11.3 [3, 4] comprehensive collections of recently created structures can be found in pertinent reviews [2],... 

Fig. 11.3. A representative synthetic barrel-stave pore self-assembled from four p-octiphenyl monomers (left). Molecular model with p-octiphenyl staves in grey, j -sheet hoops in yellow, external fullerene ligands in gold and an internal a-helix blocker in red (right, adapted from Ref. 4).

Fig. n.13. Dynamic supramolecular polymorphism of synthetic ion channels and pores including unimolecular (Ab), barrel-stave (Bb) and micellar motifs (Bd compare Fig. 11.2, text and refs [42] and [4]). 

In models of mechanisms of pore forming peptides, how is the pore of a barrel stave different from a toroidal pore ... 

Barrel-stave model Model for pore-forming amphipathic peptides in which the polar face forms a hydrophilic pore. 

Yang, L., Harroun, T. A., et al. (2001). Barrel-stave model or toroidal model A case study on melittin pores. Biophysical Journal, 81(5), 1475-1485. 

Keller, F., Hanke, W., Trissl, D. and Bakker-Grunwald, T. (1989) Pore-forming protein from Entamoeba histolytica forms voltage- and pH-controlled multi-state channels with properties similar to those of the barrel-stave aggregates. Biochim. Biophys. Acta 982 89-93. 

Fig.1. Barrel-stave poration complexes proposed for class 11 bacteriocins. Complexes maybe formed between one or two amphiphilic peptides which oligomerize and form membrane pores and ion channels [21]...

Figure 2 Transport of ions and molecules (fiUed circles) across lipid bilaya- manbranes (orange) by (a) ion channels and pores, (b) carriers, (c) endovesiculators, and (d) fusogens (empty blue ellipse) synthetic ion channels and pores are classified according to their (a) unimolecular, (e) barrel-stave, (f) barrel-hoop, (g) banel-rosette, or (h) miceUar (toroidal) active structures.

Figure 12 Mechanism of proposed pore formation by amphiphilic alpha heUces. (a) Single hehces combine with the bilayer, (b) K the concentration monomer is sufficient, the monomers insrat in the hydrophobic core and align via the barrel-stave model or (c) via the toroidal model. (Reproduced from Ref. 57. John WUey Sons, Ltd, 2000.)...

An example of pore-forming alpha helix acting via the barrel-stave models is the toxin pardaxin. This 33-residue peptide is found in the Red Sea Moses sole (a flatfish) and nsed to repel Sharks. It is found that this peptide binds to bilayer snrfaces and forms transmembrane pores consisting of 6 3 peptides. Remarkable is the efficiency... 

A second model, referred to as the toroidal model, is fairly similar to the barrel-stave model for the first step. The toroidal model differs from the barrel-stave model in that the peptides are associated with the lipid head groups and stay bound to these head groups through the entire process, even when they are perpendicularly inserted in the lipid bilayer (Figure 12c). It has been suggested that the previously discussed magainin 2 forms transmembrane pores via the toroidal model and only four to seven peptides can form a transmembrane pores at Upidipeptide ratios of 100 ... 

Another important example of a self-assembled structure is the channel-forming molecule amphotericin (Figure 4). Amphotericin 1 is a polyene-polyol macrocylce capable of forming cation-selective channels. This molecule is well suited to form barrel stave pores. One side of the molecule presents multiple hydroxyl groups that assist cation coordination, whereas the polyene chain interacts with the hydrophobic part of the membrane. The hydrophilic groups function as a polar head group, positioning the molecule... 

The simplest design is a unimolecular ion channel. Nevertheless, designing and synthesizing such macromolecules is complicated, and numerous examples of ion channels rely in the self-assembly of several structures within the membrane to form the functional pore. Depending on the type of molecules macrocycles, staves, or even smaller molecules can form barrel-hoop, barrel staves, or barrel rosette structures (Figure 7). 

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