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1.C.18 The Melittin (Melittin) Family

Many organisms synthesize proteins (or peptides) which are degraded to relatively small hydrophobic or amphipathic, bioactive peptides. These peptides exhibit antibiotic, fungicidal, virucidal, hemolytic and/or tumoricidal activities by interacting with membranes and forming transmembrane channels that allow the free flow of electrolytes, metabolites and water across the phospholipid bilayers. Most of these peptides appear to function in biological warfare. There are many designations given to these bioactive peptides. They include the magainins, cecropins, melittins, defensins, bacteriocidins, etc. Certain common structural features observed between members of distinct families suggest that at least some of these families share a common ancestry.  The process of pore formation for mellitin in lipid bilayers has been studied in some detail (Lee et al. 2013).  Melittin (26 residues) is possibly the best studied of the insect peptide toxins. It is found in the venom of the European honey bee, Apis mellifera. Three-dimensional structures of melittin have been elucidated.  The ohmic behavior of melittin is explained by the persistence of the peptide orientation initially assumed at trans-negative potentials, even after application of trans-positive ones (Becucci et al. 2016). The interactions of melittin with the outer and cytoplasmic membranes of live E. coli cells has been studied (Yang et al. 2018).

Time-dependent pore formation has been studied in individual giant unilamellar vesicles exposed to a melittin solution (Lee et al., 2008). An individual vescile first expanded its surface area at constant volume and then suddenly reversed expansion of its volume at constant area. The area expansion, the volume expansion, and the point of reversal all matched the results of equilibrium measurements performed on peptide-lipid mixtures. The mechanism includes negative feedback that makes peptide-induced pores stable with a well defined size. Melittin creates transient pores in lipid bilayers (Santo et al. 2013; Wiedman et al. 2013). It and its derivatives penetrate and form water channels in bacterial and mammalian cell membranes (Wu et al. 2016). The energetics of the process involving addition of melittin to a membrane of known composition to form a transmembrane pore have been estimated (Lyu et al. 2017).

Sengupta et al. (2008) used molecular dynamics simulation to study the interaction of a specific class of melittin with a dipalmitoylphosphatidylcholine bilayer. Transmembrane pores spontaneously formed above a critical peptide to lipid ratio. The lipid molecules bent inwards to form a toroidally shaped pore but with only one or two peptides lining the pore, in contrast to the traditional models of toroidal pores in which the peptides are assumed to adopt a transmembrane orientation. Sengupta et al., 2008 reported that peptide aggregation, either prior to or after binding to the membrane surface, is a prerequisite for pore formation, but that the presence of a stable helical secondary structure of the peptide is not. Electrostatic interactions are important in the poration process; removing charges of the basic amino-acid residues of melittin prevents pore formation. In the absence of counter ions, pores not only form more rapidly, but lead to membrane rupture via a novel recursive poration pathway.

A 9-mus all-atom molecular-dynamics simulation starting from a closely packed transmembrane melittin tetramer in DMPC showed formation of a toroidal pore after 1 mus (Leveritt et al. 2015). The pore remains stable with a roughly constant radius for the rest of the simulation. One or two melittin monomers frequently transitioned between transmembrane and surface states. All four peptides were largely helical. A simulation in a DMPC/DMPG membrane did not lead to a stable pore, consistent with the experimentally observed lower activity of melittin in anionic membranes. Thus, a dynamic toroidal pore seems to account for the transport properties of melittin (Leveritt et al. 2015). Melittin can form small short-lived pores and larger more stable pores (Sun et al. 2015).  The mecnanism of melittin action has been reviewed (Hong et al. 2019). Conformational changes and inter-peptide cooperation of the peptide, as well as melittin-induced disturbances to membrane structure, such as deformation and lipid extraction, were regarded as key factors influencing membrane insertion. The associated intermediate states in peptide conformations, lipid arrangements, membrane structure, and mechanical properties during this process were specifically considered. Potential strategies for enhancing the poration ability and improving the antimicrobial performance of AMPs are included as well (Hong et al. 2019).

Melittin peptides in lipid bilayers adopt either a transmembrane conformation or a U-shaped conformation, which are referred to as T- and U-peptides, respectively. Several U-peptides and/or T-peptides aggregate to form stable pores. Sun et al. 2022 analyzed a T-pore consisting of four T-peptides and a U-pore consisting of three U-peptides and one T-peptide. In both pores, peptides are organized in a manner such that polar residues face inward and hydrophobic residues face outward, which stabilizes the pores and produces water channels. Compared with the U-pore, the T-pore has lower energy, larger pore diameter, and higher permeability. However, the T-pore occurs less frequently than the U-pore, probably because the formation of the T-pore is kinetically slower than that of the U-pore. The stability and permeability of both pores were confirmed (Sun et al. 2022).

The generalized transport reaction catalyzed by channel-forming amphipathic peptides is:

Small solutes, electrolytes and water (in)   small solutes, electrolytes and water (out)

This family belongs to the: Cecropin Superfamily.

References associated with 1.C.18 family:

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Bechinger, B., M. Zasloff and S.J. Opella (1993). Structure and orientation of the antibiotic peptide magainin in membrane by solid-state nuclear magnetic resonance spectroscopy. Prot. Sci. 2: 2077-2084. 8298457
Becucci, L., G. Aloisi, and R. Guidelli. (2016). When and how the melittin ion channel exhibits ohmic behavior. Bioelectrochemistry 113: 51-59. [Epub: Ahead of Print] 27732908
Ganz, T., J.R. Rayner, E.V. Valore, A. Tumolo, K. Talmadge and F. Fuller (1989). The structure of the rabbit macrophage defensin genes and their organ-specific expression. J. Immunol. 143: 1358-1365. 2745983
Gudmundsson, G.H., D.A. Lidholm, B. Asling, R.B. Gan and H.G. Boman (1991). The cecropin locus–cloning and expression of a gene cluster encoding 3 antibacterial peptides in Hyalophora cecropia. J. Biol. Chem. 266: 11510-11517. 1711035
Hill, C.P., J. Yee, M.E. Selsted and D. Eisenberg (1991). Crystal structure of defensin HNP-3, an amphiphilic dimer: mechanisms of membrane permeabilization. Science 251: 1481-1485. 2006422
Hong, J., X. Lu, Z. Deng, S. Xiao, B. Yuan, and K. Yang. (2019). How Melittin Inserts into Cell Membrane: Conformational Changes, Inter-Peptide Cooperation, and Disturbance on the Membrane. Molecules 24:. 31067828
Kourie, J.I. and A.A. Shorthouse (2000). Properties of cytotoxic peptide-formed ion channels. Am. J. Physiol. Cell Physiol. 278: C1063-C1087. 10837335
Lee MT., Sun TL., Hung WC. and Huang HW. (2013). Process of inducing pores in membranes by melittin. Proc Natl Acad Sci U S A. 110(35):14243-8. 23940362
Lee, M.T., W.C. Hung, F.Y. Chen, and H.W. Huang. (2008). Mechanism and kinetics of pore formation in membranes by water-soluble amphipathic peptides. Proc. Natl. Acad. Sci. USA 105: 5087-5092. 18375755
Leveritt, J.M., 3rd, A. Pino-Angeles, and T. Lazaridis. (2015). The structure of a melittin-stabilized pore. Biophys. J. 108: 2424-2426. 25992720
Lyu, Y., N. Xiang, X. Zhu, and G. Narsimhan. (2017). Potential of mean force for insertion of antimicrobial peptide melittin into a pore in mixed DOPC/DOPG lipid bilayer by molecular dynamics simulation. J Chem Phys 146: 155101. 28433027
Matsuzaki, K. (1998). Magainins as paradigm for the mode of action of pore forming polypeptides. Biochim. Biophys. Acta 1376: 391-400. 9804997
Nagaoka, I., A. Someya, K. Iwabuchi and T. Yamashita (1991). Characterization of cDNA clones encoding guinea pig neutrophil cationic peptides. FEBS Lett. 280: 287-291. 2013325
Pardi, A., X.L. Zhang, M.E. Selsted, J.J. Skalicky and P.F. Yip (1992). NMR studies of defensin antimicrobial peptides. 2. Three-dimensional structures of rabbit NP-2 and human HNP-1. Biochemistry 31: 11357-11364. 1445873
Pino-Angeles, A. and T. Lazaridis. (2018). Effects of Peptide Charge, Orientation, and Concentration on Melittin Transmembrane Pores. Biophys. J. 114: 2865-2874. 29925023
Santo KP., Irudayam SJ. and Berkowitz ML. (2013). Melittin creates transient pores in a lipid bilayer: results from computer simulations. J Phys Chem B. 117(17):5031-42. 23534858
Sengupta, D., H. Leontiadou, A.E. Mark, and S.J. Marrink. (2008). Toroidal pores formed by antimicrobial peptides show significant disorder. Biochim. Biophys. Acta. 1778: 2308-2317. 18602889
Sun, D., J. Forsman, and C.E. Woodward. (2015). Multistep Molecular Dynamics Simulations Identify the Highly Cooperative Activity of Melittin in Recognizing and Stabilizing Membrane Pores. Langmuir 31: 9388-9401. 26267389
Sun, L., S. Wang, F. Tian, H. Zhu, and L. Dai. (2022). Organizations of melittin peptides after spontaneous penetration into cell membranes. Biophys. J. [Epub: Ahead of Print] 36199252
Terwilliger, T.C. and D. Eisenberg (1982). The structure of melittin. II. Interpretation of the structure. J. Biol. Chem. 257: 6016-6022.
Vlasak, R., C. Unger-Ullmann, G. Kreil and A.M. Frischauf (1983). Nucleotide sequence of cloned cDNA coding for honeybee prepromelittin. Eur. J. Biochem. 135: 123-126. 6309516
Wiedman, G., K. Herman, P. Searson, W.C. Wimley, and K. Hristova. (2013). The electrical response of bilayers to the bee venom toxin melittin: evidence for transient bilayer permeabilization. Biochim. Biophys. Acta. 1828: 1357-1364. 23384418
Wu, X., A.K. Singh, X. Wu, Y. Lyu, A.K. Bhunia, and G. Narsimhan. (2016). Characterization of antimicrobial activity against Listeria and cytotoxicity of native melittin and its mutant variants. Colloids Surf B Biointerfaces 143: 194-205. 27011349
Yang, Z., H. Choi, and J.C. Weisshaar. (2018). Melittin-Induced Permeabilization, Re-sealing, and Re-permeabilization of E. coli Membranes. Biophys. J. 114: 368-379. 29401434
Zhang, X.L., M.E. Selsted and A. Pardi (1992). NMR studies of defensin antimicrobial peptides. 1. Resonance assignment and secondary structure determination of rabbit NP-2 and human HNP-1. Biochemistry 31: 11348-11356. 1445872