Similarities between sweet protein thaumatin and a pathogenisis-related protein from tobacco

A pathogenesis-related protein, PR-R from virus-infected tobacco leaves, has been compared in immunological, electrophoretic and chromatographic properties with the sweet protein thaumatin with which it is believed to have ∼ 65% homology of amino acid sequence. PR-R runs faster in PAGE after it has been reduced, and both proteins run slower in SDS PAGE when reduced. PR-R and thaumatin eluted from Superose 12. FPLC columns with apparent Mrs of 14 500 and 12 000 respectively, i.e. 68 and 55% of expected values: following reduction and S-carboxymethylation thaumatin eluted with an apparent Mr of 48 000. Neither protein in the native state reacts with polyclonal antisera produced against the other protein. Following reduction, cross-reactions can be demonstrated in immunoblots. The results implicate the intramolecular disulphide bridges of both proteins in conferring a compact configuration on the native proteins, rendering some common epitopes inaccessible.