Circular proteins in plants - Solution structure of a novel macrocyclic trypsin inhibitor from Momordica cochinchinensis

Much interest has been generated by recent reports on the discovery of circ ular (i.e. head-to-tail cyclized) proteins in plants. Here we report the th ree-dimensional structure of one of the newest such circular proteins, MCoT I-II, a novel trypsin inhibitor from Momordica cochinchinensis, a member of the Cucurbitaceae plant family. The structure consists of a small beta -sh eet, several turns, and a cystine knot arrangement of the three disulfide b onds. Interestingly, the molecular topology is similar to that of the plant cyclotides (Craik, D. J., Daly, N. L., Bond, T., and Waine, C. (1999) J. M ol. Biol, 294, 1327-1336), which derive from the Rubiaceae and Violaceae pl ant families, have antimicrobial activities, and exemplify the cyclic cysti ne knot structural motif as part of their circular backbone. The sequence, biological activity, and plant family of MCoTI-II are all different from kn own cyclotides. However, given the structural similarity, cyclic backbone, and plant origin of MCoTI-II, we propose that MCoTI-II can be classified as a new member of the cyclotide class of proteins. The expansion of the cycl otides to include trypsin inhibitory activity and a new plant family highli ghts the importance and functional variability of circular proteins and the fact that they are more common than has previously been believed, Insights into the possible roles of backbone cyclization have been gained by a comp arison of the structure of MCoTI-II with the homologous acyclic trypsin inh ibitors CMTI-I and EETI-II from the Cucurbitaceae plant family.