Journal
PROTEIN SCIENCE
Volume 15, Issue 2, Pages 248-257Publisher
WILEY
DOI: 10.1110/ps.051807706
Keywords
Bcl-2 family; Bcl-X-L; solution to membrane conformational change; diphtheria toxin; pH-dependence; pore-forming toxins; protein folding
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Funding
- NCRR NIH HHS [S10 RR020922, S10 RR020922-01] Funding Source: Medline
- NIGMS NIH HHS [R01 GM067180, GM067180] Funding Source: Medline
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Regulation of programmed cell death by Bcl-x(L) is dependent on both its solution and integral membrane conformations. A conformational change from solution to membrane is also important in this regulation. This conformational change shows a pH-dependence similar to the translocation domain of diphtheria toxin, where an acid-induced molten globule conformation in the absence of lipid vesicles mediates the change from solution to membrane conformations. By contrast, Bcl-x(L)Delta TM in the absence of lipid vesicles exhibits no gross conformational changes upon acidification as observed by near- and far-UV circular dichroism spectropolarimetry. Additionally, no significant local conformational changes upon acidification were observed by heteronuclear NMR spectroscopy of Bcl-x(L)Delta TM. Under conditions that favor the solution conformation (pH 7.4), the free energy of folding for Bcl-X-L Delta TM (Delta G degrees) was determined to be 15.8 kcal(.)mol(-1). Surprisingly, under conditions that favor a membrane conformation (pH 4.9), Delta G degrees was 14.6 kcal (.) mol(-1). These results differ from those obtained with many other membrane-insertable proteins where acid-induced destabilization is important. Therefore, other contributions must be necessary to destabilize the solution conformation Bcl-x(L) and favor the membrane conformation at pH 4.9. Such contributions might include the presence of a negatively charged membrane or an electrostatic potential across the membrane. Thus, for proteins that adopt both solution and membrane conformations, an obligatory molten globule intermediate may not be necessary. The absence of a molten globule intermediate might have evolved to protect Bcl-x(L) from intracellular proteases as it undergoes this conformational change essential for its activity.
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