REVERSIBLE DENATURATION, SELF-AGGREGATION, AND MEMBRANE-ACTIVITY OF ESCHERICHIA-COLI ALPHA-HEMOLYSIN, A PROTEIN STABLE IN 6-M-UREA

Escherichia coli alpha-hemolysin (HlyA) is an extracellular protein to xin (107 kDa) whose cell lytic activity may be preserved for months at -20 degrees C in the presence of 6 M urea, although it decays rapidly in urea-free buffers. This paper describes experiments addressed to u nravel the role of urea in HlyA stabilization. Urea up to 8 M inhibits the Ca2+-binding and hemolytic activities of the protein, alters its secondary and tertiary structures, and reduces its tendency to self-ag gregation. All these changes are largely reversed upon urea removal by dilution or dialysis, suggesting that they are interrelated. Furtherm ore, the extent of recovery of the native activities and structural fe atures of cl-hemolysin that follows urea removal increases with the co ncentration of urea during the previous phase. Thus, it seems that ure a elicits the reversible transition of HlyA to a less active but more stable state whose structure differs significantly from that of the na tive protein. Moreover dialysis equilibration of the protein with buff ers containing 3 M urea induces the formation of a molecular form of H lyA 5-10 times more active than the native protein in the absence of u rea. This hyperactive intermediate appears to keep the native secondar y structure of HlyA, but with a less compact tertiary structure, that increases the number of exchangeable Ca2+ ions under these conditions. Changes in the intrinsic fluorescence of HlyA also support the notion of a conformational change in the high-activity intermediate. The int ermediate is only detected when assayed in the presence of Ca2+ and 3 M urea and can bind a large number of calcium ions (approximate to 12 vs approximate to 3 for the native protein); it shows a large tendency to self-aggregation and presumably, in the presence of membranes, a s imilar tendency to irreversible insertion, which may be the reason for its high lytic activity.