THIOREDOXIN ACTIVITY IN THE C-TERMINUS OF PHALARIS S-PROTEIN

Self-incompatibility in the grass Phalaris coerulescens is controlled by two genes S and Z. Isolation and sequencing of two S alleles showed that they encode proteins that are highly conserved at the C terminus with significant homology to thioredoxin H proteins. In particular, t he residues in and around the active site of thioredoxin, Trp-Cys-Gly- Pro-Cys, are perfectly conserved. The C terminus of the S protein has been expressed in Eschericia coli and purified to homogeneity on Ni-NT A resin. Functional assays showed that the protein has thioredoxin act ivity; it can act as a substrate for E. coli thioredoxin reductase and also catalyse the reduction of insulin by dithiothreitol. The possibl e role of thioredoxin-like activity of the S protein in mediating the incompatibility reaction in Phalaris is discussed.