title: On the nature of the unfolded intermediate in the in vitro transition of the colicin E1 channel domain from the aqueous to the membrane phase.
abstract: The transition of the colicin E1 channel polypeptide from a water-soluble to membrane-bound state occurs in vitro at acid pH values that are associated with an unfolded channel structure whose properties qualitatively resemble those of a "molten globule," or "compact unfolded," intermediate state. The role of such a state for activity was tested by comparing the pH dependence of channel-induced solute efflux and the amplitude of the near-UV CD spectrum. The requirement of a partly unfolded state for activity was shown by the coincidence of the onset of channel activity measured for 4 different lipid compositions with the decrease in near-UV CD amplitude as a function of pH. Tertiary constraints on the 3 tryptophans of the colicin channel, assayed by the amplitude of the near-UV CD spectrum, are retained over the pH range 3-4 where channel activity could be measured and, as well, at pH 2. In addition, the tryptophan fluorescence emission spectrum is virtually unchanged over the pH range 2-6. The temperature independence of the near-UV spectrum at pH 3-6 up to 70 degrees C implies that the colicin E1 channel polypeptide is more stable than that of colicin A. A transition between 53 and 58 degrees C in the amplitude of the near-UV CD is consistent with preservation of part of the hydrophobic core in a destabilized state at pH 2. Thus, the unfolded state associated with colicin activity at acidic pH has the properties of a "compact unfolded" state, having some, but not all of the properties of a "molten globule."(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: S L
authors.initials: SL
authors.lastname: Schendel
authors.forename: W A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D018118:Chloride Channels / Q000737:chemistry*; D002942:Circular Dichroism; D003087:Colicins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008081:Liposomes; D008565:Membrane Proteins / Q000737:chemistry*; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D012996:Solutions; D013056:Spectrophotometry, Ultraviolet; D014867:Water
chemical_list: D018118:Chloride Channels; D003087:Colicins; D008081:Liposomes; D008565:Membrane Proteins; D012996:Solutions; D014867:Water