Native melittin with its C-terminal amide, calculated with the residue masses and pKa set used on this site.
The main component of bee venom
Melittin makes up roughly half of the dry weight of honeybee (Apis mellifera) venom, with reported values between about 40% and 60%. It is responsible for much of the immediate pain of a sting. The venom also contains phospholipase A2, hyaluronidase, the peptide apamin, mast cell degranulating peptide, and small molecules such as histamine. Melittin and phospholipase A2 act together: melittin disrupts membranes, which makes their phospholipids more accessible to the enzyme.
Allergy to bee stings is caused mainly by other venom components. Phospholipase A2, designated Api m 1, is the major allergen; melittin itself, Api m 4, is a comparatively minor one.
Structure
Melittin is a 26-residue peptide with a C-terminal amide. Its sequence is strikingly divided. The first 20 residues are mostly hydrophobic, with a single lysine at position 7; the last six, KRKRQQ, form a highly cationic tail. The native peptide contains no acidic residues, and with its C-terminus amidated it has no acidic groups at all. It carries a net charge of about +6 at physiological pH and, like substance P, has no true isoelectric point: its charge stays positive at every pH.
In water at low concentration melittin is mostly unstructured. In membranes, in helix-inducing solvents, and at high concentrations or ionic strength, it forms an α-helix with a kink at proline 14, giving two helical segments at an angle of roughly 120° to each other. In the crystal structure, solved by Terwilliger and Eisenberg in 1982, four helices associate into a tetramer with their hydrophobic faces buried in the middle and their charged faces outside, an arrangement also observed in concentrated, salty solutions.
Because of this conditional folding, melittin is a standard example in discussions of peptide secondary structure. Its hydropathy profile, with a hydrophobic N-terminal region and a sharp drop at the cationic tail, is the worked example in the guide to reading a hydropathy plot and is shown on the home page of this site.
Biosynthesis
Melittin is made in the venom gland as a 70-residue precursor, prepromelittin. After removal of the signal peptide, the pro-region is trimmed from the N-terminus two residues at a time by a dipeptidyl aminopeptidase. The pro-region is built from repeating dipeptide units that the enzyme recognises, and the process stops when it reaches the mature sequence. The C-terminal amide is produced from a glycine extension by the amidating enzyme, as for most amidated peptides described in the guide to post-translational modifications. The inactive precursor protects the bee's own tissues until the peptide is secreted.
How melittin attacks membranes
Melittin binds strongly to lipid bilayers, where it folds into its amphipathic helix. Its behaviour depends on how much peptide is present relative to the lipid:
- At low peptide-to-lipid ratios, melittin lies parallel to the membrane surface, with its hydrophobic face inserted among the lipid acyl chains and its charged face at the head groups. This thins and strains the membrane.
- Above a threshold ratio, the peptides reorient and form pores, generally described as toroidal pores in which lipid head groups line the pore together with the peptide.
- At high concentrations, membranes can break up in a detergent-like manner.
These mechanisms are described in more detail in the guide to antimicrobial peptides. Unlike selective antimicrobial peptides, melittin is not deterred by the zwitterionic, cholesterol-containing membranes of animal cells. It lyses red blood cells at low micromolar concentrations, and haemolysis by melittin is often used as a positive control in toxicity assays for other peptides.
Melittin with the tools on this site
| Property | Linear free acid (calculator input) | Native, C-terminal amide |
|---|---|---|
| Average MW | 2847.46 | 2846.48 |
| Monoisotopic mass | 2845.7381 | 2844.7541 |
| Net charge at pH 7.4 | about +4.8 | about +5.8 |
| Isoelectric point | about 12.5 | None; positive at all pH values |
| GRAVY | +0.27 | |
| Extinction coefficient, 280 nm | 5,500 M⁻¹cm⁻¹ (one Trp) | |
The single tryptophan at position 19 has a second use. Its fluorescence changes when the peptide moves from water into a membrane, because the indole ring enters a less polar environment. Tryptophan fluorescence is therefore a standard way to follow melittin binding to lipid vesicles.
Melittin is also a convenient test case for isotope patterns in mass spectrometry. At about 2.8 kDa, its monoisotopic peak is no longer the tallest in a resolved spectrum; the M+1 peak is, as explained in the guide to molecular weight.
Melittin in research
Melittin has been studied for decades as a model for how peptides interact with lipid bilayers, and it is widely used in membrane biophysics to calibrate methods for studying pore formation.
Its potent, non-selective cytotoxicity has also attracted interest in cancer and antimicrobial research. Free melittin is too haemolytic for systemic use, so research has focused on ways to direct it: attaching it to nanoparticles, engineering hybrid peptides that retain antibacterial activity with less haemolysis, and conjugating it to targeting molecules. These approaches remain experimental. They illustrate a general problem in peptide therapeutics described in the antimicrobial peptides guide: activity against bacterial or tumour membranes and toxicity to healthy cells often rise together.
Frequently asked questions
Is melittin an antimicrobial peptide?
It is strongly antibacterial and has the typical AMP profile of positive charge and amphipathicity, but it is not selective: it damages animal cells as readily as bacteria. It is usually classed as a venom toxin and a model membrane-lytic peptide.
Why does a bee sting hurt?
Mainly because melittin damages cell membranes and activates pain-sensing nerve endings, both directly and by triggering the release of inflammatory mediators. Other venom components contribute to swelling and inflammation.
Why is melittin helical in membranes but not in water?
In water, a single peptide gains too little from folding to overcome the loss of flexibility. In a membrane, the helix lets the hydrophobic residues insert into the lipid layer and satisfies the backbone hydrogen bonds away from water, which makes folding strongly favourable.
What does the proline kink do?
Proline 14 breaks the helix into two segments at an angle. Studies with analogues in which the proline is replaced suggest that the kink influences how melittin inserts into membranes and how strongly it lyses cells.
References
- Habermann E (1972) Bee and wasp venoms. Science 177:314–322.
- Terwilliger TC, Eisenberg D (1982) The structure of melittin. II. Interpretation of the structure. Journal of Biological Chemistry 257:6016–6022.
- Kreil G, Haiml L, Suchanek G (1980) Stepwise cleavage of the pro part of promelittin by dipeptidylpeptidase IV. European Journal of Biochemistry 111:49–58.
- Raghuraman H, Chattopadhyay A (2007) Melittin: a membrane-active peptide with diverse functions. Bioscience Reports 27:189–223.