Calorimetric investigations of the structural stability and interactions of colicin B domains in aqueous solution and in the presence of phospholipid bilayers.
The effects of pH and temperature on the stability of interdomain interactions of colicin B have been studied by differential-scanning calorimetry, circular dichroism, and fluorescence spectroscopy. The calorimetric properties were compared with those of the isolated pore-forming fragment. The unfolding profile of the full-length toxin is consistent with two endothermic transitions. Whereas peak A (T(m) = 55 degrees C) most likely corresponds to the receptor/translocation domain, peak B (T(m) = 59 degrees C) is associated with the pore-forming domain. By lowering the pH from 7 to 3.5, the transition temperature of peaks A and B are reduced by 25 and 18 degrees C, respectively, due to proton exchange upon denaturation. The isolated pore-forming fragment unfolds at much higher temperatures (T(m) = 65 degrees C) and is stable throughout a wide pH range, indicating that intramolecular interactions between the different colicin B domains result in a less stable protein conformation. In aqueous solution circular dichroism spectra have been used to estimate the content of helical secondary structure of colicin B ( approximately 40%) or its pore-forming fragment ( approximately 80%). Upon heating, the ellipticities at 222 nm strongly decrease at the transition temperature. In the presence of lipid vesicles the differential-scanning calorimetry profiles of the pore-forming fragment exhibit a low heat of transition multicomponent structure. The heat of transition of membrane-associated colicin B (T(m) = 54 degrees C at pH 3.5) is reduced and its secondary structure is conserved even at intermediate temperatures indicating incomplete unfolding due to strong protein-lipid interactions.
Ortega
A
A
Departamento de Bioquimica, Facultad de Medicina, Universidad Nacional Autónoma de México, México City 04510, Mexico.
Lambotte
S
S
Bechinger
B
B
D002151:Calorimetry; D002942:Circular Dichroism; D003087:Colicins / Q000737:chemistry* / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008051:Lipid Bilayers / Q000378:metabolism*; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013816:Thermodynamics
D003087:Colicins; D008051:Lipid Bilayers