Membrane pore-forming peptides

Emphasis is placed here on features of the biological membranes which are implicated in substrate transport. The lipid bilayer in the "gel" state, in the absence of additives, forms an effective barrier against polar ions and water soluble substrates. Changing the fluidity, by phase transition (induced by temperature changes and/or by the addition of foreign ions or molecules) or by the incorporation of additives (cholesterol, for example), profoundly influences the structure and, hence, the transport properties of membranes. This, and the presence of channel or pore forming peptides or proteins, opens the door to a number of transport mechanisms which will be summarized in the following section. 

Other scientific interest surrounds a derivative of kiwellin called kissper, also an extract from kiwi. Kissper is a member of a new class of pore-forming peptides that influence cell membrane porosity having potential for biotechnology developments in drug delivery. 

The use of microsomes along with UDPGA as a cofactor assay to measure UGT enzyme activity has been hampered historically by the fact that this enzymatic activity in microsomes is often in a latent form and requires activation by physical or detergent-induced disruption of the membrane matrices. Recently, a generic method involving the addition of the pore-forming peptide alamethicin to overcome the latency exhibited by this enzyme system has been described.99 The inclusion of alamethicin seems to provide a more consistent method of assessing UGT enzyme activity. 

Recent studies that reported incorporation of pore-forming peptides and proteins into BLMs composed of poly(lipids) were discussed in Sect. 3. In both of these examples, a conformational change was not required for channel activity, and the bilayer was not completely polymerized. Reconstitution of TMPs into solid- and polymer-supported membranes composed solely of polymerized lipids has been reported by two groups in recent years. 

What is the difference between a membrane permeant peptide and a pore-forming peptide ... 

In the case of human microsome samples, pretreatment of the protein with alamethicin, a pore-forming peptide, is known to increase enzyme activity (II). This reagent is excluded from the SN-38 glucuronidation assay presented here because alamethicin has almost no effect on the glucuronidation activity of the COS-1 membrane fractions. 

They may enter the cytosol and fold quickly into a compact form. This may require only a few seconds, whereas the translation process in the ribosome may take many seconds. The folding will therefore be cotranslational.525 Depending upon the N-terminal signal peptide the protein may later unfold and pass through a membrane pore or translocon into the endoplasmic reticulum (ER), a mitochondrion, chloro-plast, or peroxisome. Wherever it is, it will be crowded together with thousands of other proteins. It will interact with many of these, and evolution will have enabled some of these to become chaperones (discussed in Chapter 10).526... 

Once internalized, the essential step for the polyplex is to escape rapidly the endosomal vesicle in order to release the nucleic acid in the cytosol and prevent its lysosomal degradation. As the endosomal and lysosomal pH presents values between 4.5 and 6.5 and therefore differs from the neutral pH of 7.4 in other biological compartments [58], some polycations containing protonable residues like PEI facilitate this step by the proton sponge effect [59, 60]. As not all cationic polymers display this attribute, another effective method for enhanced endosomal polyplex release is incorporation of specific endosomal membrane disrupting or pore-forming domains, such as lytic lipid moieties or endosomolytic peptides. 

As stated, biological membranes are normally arranged as bilayers. It has, however, been observed that some lipid components of biological membranes spontaneously form non-lamellar phases, including the inverted hexagonal form (Figure 1.9) and cubic phases [101]. The tendency to form such non-lamellar phases is influenced by the type of phospholipid as well as by inserted proteins and peptides. An example of this is the formation of non-lamellar inverted phases by the polypeptide antibiotic Nisin in unsaturated phosphatidylethanolamines [102]. Non-lamellar inverted phase formation can affect the stability of membranes, pore formation, and fusion processes. So-called lipid polymorphism and protein-lipid interactions have been discussed in detail by Epand [103]. 

In the larger peptide antibiotic compounds comprising the class of lantibiotics, the shape of the molecule is determined by several cyclic peptides, including two annulated peptide rings, present within one molecule, giving the lantibiotic a unique way to interact with the target molecule lipid II and subsequent pore-forming capabilities in phospholipid membranes [2]. 

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.

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