Similarity of protein conformation at low pH and high temperature observed for B-chains of two plant toxins: ricin and mistletoe lectin 1.

A comparative study of subunits of two plant toxins, ricin (RC) and mistletoe lectin 1 (ML 1), has been undertaken. The study suggests that isolated B-chains of these toxins undergo structural transitions at low pH (from 5 to 4) and high temperature (45 degrees C), and as a result of the guanidine hydrochloride denaturing effect (to 3 M). Our results indicate that the protein conformation observed at low pH and high temperature are similar, though not identical. These conformations differ from the native one (pH 7, 25 degrees C), the protein in these conformations has a low fluorescence tryptophan intensity, and tryptophans are more exposed to aqueous solutions. However, these conformations differ also from the state unfolded by guanidine hydrochloride. An assumption is made that the partially denatured protein structure, exhibited at low pH and high temperature, is a functionally essential intermediate state of the toxin B-chain.