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All of the original discoveries of cyclotides were the result of their natural activities - either through native medicinal use or as the result of various bioassays. The cyclotides exhibit a range of interesting bioactivities, apart from their uterotonic activity, they also display haemolytic [42,23,21,56,22,55], anti-HIV [22,24,29], neurotensin inhibitory , anti-microbial , insecticidal  and anti-tumour/cytotoxic activities [57,58]. Although the cyclotides have been shown to possess this range of activities their functional significance to the plant has not been definitively established, however their potent insecticidal activity would seem to suggest that they have a role in plant defences. The natural activities of the cyclotides are summarised in Table 1.1.
|circulin A and B
|circulin A and B
|circulin A and B
The mechanisms by which these bioactivities are mediated is not known, although the unusual hydrophobicity of the cyclotides may be involved in some of them. Certainly haemolytic, anti-microbial and cytotoxic activity could be the result of hydrophobic interactions with the plasma membrane -- whether by the formation of pores or some other action. Other studies have hinted, however, that the cyclotides may mediate specific interactions with a variety of targets. For example, it has been speculated that cyclopsychotride A operates via a receptor mediated mechanism to inhibit the binding of neurotensin to human HT-29 carcinoma cells, although the receptor that may be involved has not determined . Furthermore, anti-microbial assays have suggested that specific cyclotides have different effectiveness against particular bacteria, with kalata B1 and circulin A showing effectiveness against the Gram-positive Staphylococcus aureus while circulin A and cyclopsychotride A acted against both Gram-positive and Gram-negative bacteria . Removal of the positive charge in kalata B1 reduced it's anti-microbial activity leading to the suggestion that electro-static interactions are important for this activity . Finally, a novel mode of action has been suggested to be involved in the cytotoxic activity shown by varv A, varv F and cycloviolacin O2 , as based on the significant differences between the activity profiles of these cyclotides relative to other conventional anti-tumour medications .
Feeding trials of kalata B1 in the crop pest Helicoverpa armigera showed potent inhibition of larval growth suggesting a role for the cyclotides in plant defences . It is possible that the cyclotides may be acting to inhibit key digestive enzymes. Such a strategy is common in the plant kingdom, with protein inhibitors of proteases widely distributed and their operation against exogenous proteases indicating that they have an important role in plant defences [59,60,61,62]. However, assays conducted in the same study using kalata B1 and B2 showed no inhibition of trypsin, chymotrypsin or -amylases. Although this does not preclude the possibility of activity against another, as yet unknown, protease, it is also possible that the cyclotides are players in a different plant defence strategy.
Given the number of cyclotides discovered and the range that exist in a single plant , it would seem likely that they possess some type of specific activity, possibly receptor mediated, to fulfil their function within the plant. This consideration is further strengthened by the marked difference in conservation between the two cyclotide subfamilies. Both types exist in the same plant and it is possible that the Möbius subfamily may perform a distinct function from that fulfilled by the bracelet subfamily. Certainly the variation of charged residues within the bracelet subfamily, especially in the solvent exposed loops 5 and 6, may point to specific receptor or membrane interactions. Genetic variation is often a trademark of defence based proteins as the ``arms'' race between the plant and the attacker drives the evolution of novel modes of action. Examples of this gene-for-gene model exist in the plant world and include the Avr and Cf genes in Cladosporium fulvum and tomato  and the coevolution of insect trypsins and plant trypsin inhibitors . Whether a similar process is driving variation within the cyclotides is not known, although given these considerations this scenario cannot be discounted.
Next: Cyclotide Evolution Up: The Cyclotides Previous: Biosynthesis Jason Mulvenna