Understanding membrane protein folding and stability is required for a molecular explanation of function and for the development of interventions in membrane protein folding diseases. Stable aqueous detergent solutions of the Escherichia coli glycerol facilitator in its native oligomeric state have been difficult to prepare as the protein readily unfolds and forms nonspecific aggregates. Here, we report a study of the structure and stability of the glycerol facilitator in several detergent solutions by Blue Native and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), circular dichroism (CD), and fluorescence. Protein tetramers were prepared in neutral dodecyl maltoside (DDM) and in zwitterionic lysomyristoylphosphatidylcholine (LMPC) detergent solutions that are stable during SDS-PAGE. Thermal unfolding experiments show that the protein is more stable in LMPC than in DDM. Tertiary structure unfolds before quaternary and some secondary structure in LMPC, whereas unfolding is more cooperative in DDM. The high stability of the protein in DDM is evident from the unfolding half-life of 8 days in 8 M urea, suggesting that hydrophobic interactions contribute to the stability. The protein unfolds readily in LMPC below pH 6, whereas the tetramer remains intact at pH 4 in DDM. At pH 4 in DDM, the protein is more sensitive than at neutral pH to unfolding by SDS and the effect is reversible. At pH 3 in DDM, the tetramer unfolds, losing its tertiary structure but retaining native helical structure which melts at significantly lower temperatures than in the native tetramer. The glycerol facilitator prepared in SDS is mainly monomeric and has 10% less R-helix than the native protein. CD suggests that it forms a condensed structure with non-native tertiary contacts highly similar to the state observed in LMPC at low pH. The implications of the results for in vitro and in vivo folding of the protein are discussed.
Osmolytes are naturally occurring molecules used by a wide variety of organisms to stabilize proteins under extreme conditions of temperature, salinity, hydrostatic pressure, denaturant concentration, and desiccation. The effects of the osmolyte trimethylamine N-oxide (TMAO) as well as the influence of detergent head group and acyl chain length on the stability of the Escherichia coli integral membrane protein glycerol facilitator (GF) tetramer to thermal and chemical denaturation by sodium dodecyl sulphate (SDS) are reported. TMAO promotes the association of the normally tetrameric α-helical protein into higher order oligomers in dodecyl-maltoside (DDM), but not in tetradecyl-maltoside (TDM), lyso-lauroylphosphatidyl choline (LLPC), or lyso-myristoylphosphatidyl choline (LMPC), as determined by dynamic light scattering (DLS); an octameric complex is particularly stable as indicated by SDS polyacrylamide gel electrophoresis. TMAO increases the heat stability of the GF tetramer an average of 10 °C in the 4 detergents and also protects the protein from denaturation by SDS. However, it did not promote re-association to the tetramer when added to SDS-dissociated protein. TMAO also promotes the formation of rod-like detergent micelles, and DLS was found to be useful for monitoring the structure of the protein and the redistribution of detergent during thermal dissociation of the protein. The protein is more thermally stable in detergents with the phosphatidylcholine head group (LLPC and LMPC) than in the maltoside detergents. The implications of the results for osmolyte mechanism, membrane protein stability, and protein-protein interactions are discussed.
Urea is well known as a denaturant of proteins, but there is also evidence that millimolar amounts of urea may in fact stabilize protein complexes. Advances in mass spectrometric analysis have given us the opportunity to test the effect of urea on several noncovalent complexes in buffered solutions. We expected to see lower charge states if folded proteins were more compact (and therefore more stable), and higher charge states if the proteins were denatured. We have found that mM urea interferes with some noncovalent interactions, and that the extent of interference depends on the specific protein complex. The difference seems to be related to the type of interactions, with weak ones, such as H-bonds, more sensitive to urea. Examples show that a quick check with urea may give some insights into protein stability in the mass spectrometer.
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