Frontiers | Structure-Activity Relationships of Insect Defensins | Chemistry
The yellow mealworm, Tenebrio molitor, harbors a symbiont that has spores and infects the fat body and ventral nerve chord of adult and larval beetles. Spores, Protozoan/ultrastructure; Symbiosis*; Tenebrio/parasitology*. Rapid bioassay to screen potential biopesticides in Tenebrio molitor larvae .. A negative relationship was identified between bacterial diversity and As such, the study of insect ecology and symbiosis represents an important source of .. Blood-feeding insects transmit numerous viruses, bacteria, protozoans and. A mite that attaches itself to a large beetle may be carried about and kept in situations . The parasites which commonly infest man are protozoans, flatworms .
This is due to highly complex samples that are limited in quantity and bioactive compounds present in trace amounts only that can easily be overlooked or only partially be identified Wiese et al. To fully unveil the diversity of peptides from such biological samples new advances in analytical chemistry, nucleotide sequencing and high-throughput drug screening are essential to aid in the discovery of novel AMPs.
Indeed, refined methodologies that combine classical chemical analysis with bioinformatics workflows have proven useful to harness the variety of peptides and expand the knowledge of natural product peptidomes Koehbach and Jackson, Though, to date the majority of identified defensin sequences was retrieved using either mass spectrometry based characterization of insect hemolymphs or cDNA cloning see references in Table 1.
Recent studies that describe the use of transcriptomes and genomes as valuable source of novel defensin sequences are significantly expanding the number of identified peptides per single study Gruber and Muttenthaler, ; Poppel et al. Although nucleotide based peptide discovery provides additional information about the biosynthetic origin of peptides it lacks information regarding potential post-translational modification such as C-terminal amidation.
Defensins are embedded in larger precursor molecules that consist of an ER signal peptide, a propeptide domain that precedes the mature peptide domain and ends with a conserved dibasic cleavage motif Gruber and Muttenthaler, Mature peptides are typically around 40 residues long and carry an overall positive net charge with infrequent reports of anionic examples Figures 1B,C ; Wen et al.
Naturally occurring insect defensin peptides with reported antimicrobial activity. Structural diversity of insect defensins. Sequence analysis of 57 peptides Table 1 illustrating the diversity of insect defensins. A All sequences share a conserved pattern of six cysteine residues. The minimal, maximal, and most abundant italic font length of individual sequence stretches are indicated.
D Sequences of the antibacterial phormicin and the antifungal heliomicin showing the disulfide connectivity of insect defensins. Secondary structural elements, i. Surface representations show negatively red and positively blue charged residues, demonstrating the overall cationic character of the surface of the peptides. They can be broadly divided into three major groups, i. Recent review articles have addressed current knowledge about groups i and ii Huang et al.
Insect defensins are defined to contain six conserved cysteines that form a typical arrangement of three disulfide bonds. However, peptides such as drosomycin contain eight cysteines, which is a conserved feature of plant defensins. Similar to peptides from plants or fungi they are hence classified as cis-defensins as opposed to trans-defensins found within vertebrate species Shafee et al. The tight arrangement of secondary structural elements is reflected in high stability against heat or proteases.
Although all insect defensins share this common structural motif their primary sequence Figure 1A as well as their spectrum of antimicrobial activity varies considerably Table 1. It is evident that the majority of tested peptides exhibits activity against gram-positive bacteria, however several peptides exhibit potent activity against gram-negative bacteria or are primarily active against fungi Table 1.
Yet, the attempt to accurately compare antimicrobial activities and relate them to the peptide sequences and secondary structures is challenging.
Importantly, there is a large variety of different pathogens that have been selected for testing of defensin activity and some peptides have only been tested for individual pathogens, e. Thus, activity spectra for these peptides need yet to be established. Further, assay conditions and concentration thresholds that are used to describe peptides as active or inactive can vary remarkably. When characterizing defensin activity it also has to be noted that some studies use AMPs devoid of cysteines as control peptides and such studies are more difficult to use for comparison.
Not at least experimental conditions such as the use of varying salt concentrations can change the activity of individual peptides dramatically and should also be considered in activity comparisons Lee et al.
With regard to structure-activity relationships, a key limitation for insect defensins is the low number of resolved three-dimensional structures.
insect tenebrio molitor: Topics by avesisland.info
Comparisons purely based on primary sequences are error-prone Grishin, and conservation within secondary and tertiary structure is higher as compared to the primary sequences Shafee et al. Currently only nine peptides have been characterized using solution NMR spectroscopy, including four antifungal, i.
Nevertheless, these studies provide valuable information about structure-activity relations for both antibacterial as well as antifungal insect defensins and shed light on structural determinants underlying biological activity.
For example mutation studies on the antifungal peptide ARD1 revealed subtle changes in hydrophobicity and cationicity to enhance the activity spectrum and increase potency Landon et al.
In an attempt to confer anti-bacterial activity onto the antifungal heliomicin which only differs from ARD1 in two positions changes within the N-terminal sequence led to a loss of antifungal activity highlighting its functional importance Lamberty et al.
The third antifungal peptide with the length of 44 amino acids is drosomycin. Compared to the other antifungal insect defensins it has an additional disulfide bond that connects the N-terminal loop to the C-terminus of the peptide.
A modeling study comparing drosomycin to other plant antifungal defensins such as RsAFP2 suggested a hydrophobic patch in which a lysine residue is embedded as key determinant for antifungal activity Landon et al. Indeed, experimental evidence verified this lysine residue while testing the functional role of charged residues for the antifungal activity of drosomycin Zhang and Zhu, The fourth antifungal insect defensin for which a structure has been resolved is the termite-specific termicin.
While exhibiting an amphiphilic character similar to drosomycin or heliomicin, the positions of hydrophilic and hydrophobic residues exposed on the surface are opposite.
Several residues including for example the two arginine residues in loop 3 were proposed as possible interacting partners involved in antifungal activity Da Silva et al. The other three antifungal defensins known to date are Gallerimycin, Gm defensing, and Gm defensin-like peptide Schuhmann et al.
It appears that multiple factors contribute to specificity toward antifungal activity involving the N-terminal portion of the peptide as well as a subtle interplay between hydrophobic and charged residues.
For the primarily antibacterial defensins only five available structures represent a very limited number given the large number of different peptide sequences Table 1. Additionally it is worth to mention that phormicin, sapecin, and lucifensin only differ by individual amino acids and thus it is not surprising that their three-dimensional topologies are highly similar Hanzawa et al.
In an attempt to increase activity against Staphylococcus aureus a detailed study was reported using the Anopheles defensin as well as an alignment of 40 insect defensin sequences as basis for the design of 45 peptide mutants Landon et al. The anemone is often found attached to the shell in which the hermit crab lives. In their long history hermit crabs have developed the habit of sheltering within the empty shells of mollusks such as periwinkles and whelks.
The hind portion of the has lost its hard covering and would otherwise be unprotected. As the crab gets bigger it outgrows its shelter and so has to find a new one. Often, a sea anemone attaches itself to the crab's shelter and it may envelop part of the crab's own shell as well.
The growth of the crab and anemone keep pace with each other and the crab has no need to change its shell — more and more of its is sheltered by the anemone. As the crab moves about in search of food the anemone is brought into contact with a greater supply of food and the crab is protected by the anemone's stinging cells. Symbiosis involving microorganisms Many protozoans and single-celled algae live symbiotically with animals.
Symbiotic plant cells are particularly common in planktonic-shelled protozoans — the foraminiferans and radiolarians — and in corals and other many-celled animals in tropical seas. It is possible that such associations have arisen because of the relative lack of minerals in the surface waters of warmer seas. Radiolarians have a frothy layer of cytoplasm outside the main mass of cytoplasm. Within the froth are embedded a number of tiny plants. These obtain shelter and have a ready supply of food in the form of the waste materials that the radiolarians produce.
The oxygen that the plant cells release is available to the radiolarians and possible food substances as well. By using up the waste materials alone the plants render a useful service to the animals. Many cnidarians and some flatworms have green algae Zoochlorellae living in their tissues. Hydra viridis, a cnidarian commonly found in freshwater, owes its green color to the many algal cells in its tissues.
Corals particularly reef corals and sea anemones also have symbionts in their tissues. A most interesting association is between the green alga Carteria and the flatworm Convoluta.
When very young, the latter lives the life of a normal flatworm, feeding in the same way as other free-living forms. However, at an early stage it obtains a stock of symbionts, loses its gut and becomes completely dependent on them for its food supply. The symbionts obtain a supply of carbon dioxide and nitrogen-containing waste materials.Termite symbiosis: Internal guests digest cellulose
They are also brought out into the light by the animal at appropriate times. The flatworm is supplied with oxygen and food and has its waste materials removed.
Symbiotic microorganisms — bacteriafungi yeastsand protozoans — play an important role in the lives of many insects. They may be harbored in the gut or in special cells mycetocytes which are often grouped together to form organs called mycetomes.
Most termites have symbiotic protozoans in the hind part of the gut. These actively ingest the wood particles that the termite has eaten and break them down, releasing substances that the termites can absorb. Experiments show that the termites depend on the protozoans depend on the protozoans for much of their food, and when the latter have been removed, so that the termite has none in its gut, it loses weight rapidly and dies.
One wood-eating termite grows only when the wood on which it feeds harbors a fungus population. Some wood-eating cockroaches have protozoans and bacteria in their gut and certain scarab beetle larvae house bacteria that digest cellulose. Some biting lice and several sucking lice have mycetocytes containing bacteria, and many bugs have mycetomes that contain bacteria and yeasts. It has been shown that in some female beetles the symbionts are smeared on to the eggs from special sacs as these are laid.
The beetle grubs become infected after hatching when they eat the egg shells. Mammals harbor vast populations of bacteria in their stomachs and intestines.
Ruminants such as cows and sheep have special chambers in the stomach in which the bacteria live, feeding on cellulose in the grass on which their hosts feed. By their activities the bacteria produce simple organic acids e.
Bacteria also produce vitamins e. B12 in the gut. This may be the only supply of some of these essential nutrients.