Next: Other Cyclic Peptides Up: SFTI Previous: Evolution and Distribution
SFTI-1 is a highly unusual protein - a minimised version of the active site of a phylogenetically remote class of protein. Additionally, it has shown the clearest evidence yet for the involvement of proteases in the cyclising mechanism of plant cyclic peptides. Accordingly, the resolution of the manner in which SFTI-1 is expressed and then processed is of the utmost importance for the resolution of the cyclising mechanism as the organisation and sequence of this precursor could provide valuable clues to the cyclising mechanism.
The processing of linear precursors into the cyclic peptides demands that a proteolytic cleavage of the precursor occurs to free the N, and possibly, the C-terminal of the mature peptide for ligation. The question that is then posed is whether the cyclisation then proceeds auto-catalytically, is mediated by another enzyme or does the protease itself act in the cyclising mechanism? SFTI-1 shows that proteases are capable of catalysing the formation of peptide bonds, ie. the reversal of the reaction for which they are generally known. If Occam's Razor is applied to the problem then it is clear that the simplest method of cyclisation would be cleavage and ligation performed by the one enzyme. If that is the case then the lack of other cyclic peptides in nature implies that special considerations must be met in the linear precursor before this reaction can take place. Simple examples would include the proximity of the N and C -termini in the precursor/enzyme complex, or the requirement of particular residues at either termini of the cyclised protein. In the following section other cyclic proteins are described and once again it can be seen that the general themes of normal cellular machinery, including proteases, operating on unique precursors is continued.
Next: Other Cyclic Peptides Up: SFTI Previous: Evolution and Distribution Jason Mulvenna