ANALYSIS OF THE AMINO-ACID-SEQUENCES OF PLANT BOWMAN-BIRK INHIBITORS

Plant seeds contain a large number of protease inhibitors of animal, f ungal, and bacterial origin. One of the well-studied families of these inhibitors is the Bowman-Birk family(BBI). The BBIs from dicotyledono us seeds are 8K, double-headed proteins. In contrast, the 8K inhibitor s from monocotyledonous seeds are single headed. Monocots also have a 16K, double-headed inhibitor. We have determined the primary structure of a Bowman-Birk inhibitor from a dicot, horse-gram, by sequential ed man analysis of the intact protein and peptides derived from enzymatic and chemical cleavage. The 76-residue-long inhibitor is very similar to that of Macrotyloma axillare. An analysis of this inhibitor along w ith 26 other Bowman-Birk inhibitor domains (MW 8K) available in the SW ISSPROT databank revealed that the proteins from monocots and dicots b elong to related but distinct families. Inhibitors from monocots show larger variation in sequence. Sequence comparison shows that a crucial disulphide which connects the amino and carboxy termini of the active site loop is lost in monocots. The loss of a reactive site in monocot s seems to be correlated to this. However, it appears that this disulp hide is not absolutely essential for retention of inhibitory function. Our analysis suggests that gene duplication leading to a 16K inhibito r in monocots has occurred, probably after the divergence of monocots and dicots, and also after the loss of second reactive site in monocot s.