EVIDENCE FOR A MOLTEN GLOBULE STATE IN AN OLIGOMERIC PLANT PROTEIN

Three different mustard globulin (isoforms), purified and isolated, us ing three extraction solutions (differing in ionic strength) were stud ied for structural integrity as a function of pH. Evidence of a molten globule state (a reversible intermediary state between the native and fully denatured forms) was obtained. This phenomenon may ultimately p rove to be important in the translocation of these proteins across bio logical membranes at the time of their biosynthesis. Circular dichrois m, hydrophobic probe, fluorescence spectral scans and differential sca nning calorimetry were used to study this phenomenon. Secondary and te rtiary structures (circular dichroism (CD) data) were found to be simi lar for globulins from higher ionic strength extractions, but differen t from the globulin from distilled water extraction; however, for all three isoforms, little change in secondary structure fractions as a fu nction of pH was observed. Changes in tertiary structure (near-UV CD a nd intrinsic fluorescence data) as a function of pH were observed for all three globulin isoforms with greatest changes in tertiary structur e being seen in the acidic pH range, i.e. 3-5. In contrast, all globul ins were shown to undergo the least conformational change in the pH ra nge of 6-9. (C) 1997 Elsevier Science Ltd.