Identification of a novel four-domain member of the proteinase inhibitor II family from the stigmas of Nicotiana alata

Proteinase inhibitors (PIs) of the potato type II family have been identifi ed in a number of solanaceous species. Most family members have two PI doma ins which are specific for either chymotrypsin or trypsin. More recently fa mily members have been described with three or six repeated PI domains. Her e we describe a novel four-domain family member produced in the stigmas and leaves of the ornamental tobacco, Nicotiana alata, which has high sequence identity with a six-domain member from the same species. Both proteins are produced as precursors that enter the secretory pathway and are subsequent ly processed into a series of 6 kDa PIs. The four- and six-domain precursor proteins were isolated from immature stigmas and characterised by mass spe ctrometry which revealed that both proteins had been trimmed at the N-termi nus, at a position corresponding to the predicted signal peptide cleavage s ite. Furthermore, no post-translational modifications were apparent.