Spectroscopic characterization of the conformational adaptability of Bombyx mori apolipophorin III.
Apolipophorin III (apoLp-III) from the silkmoth, Bombyx mori, has been over-expressed in Escherichia coli, purified and characterized. Far-UV CD spectroscopic analysis revealed 65% alpha-helix secondary structure. Near-UV CD spectra obtained in buffer or complexed with dimyristoylglycerophosphocholine (DMPC), provided evidence that apoLp-III alpha-helices reorient upon interaction with lipid, indicative of a protein conformational change. In guanidine hydrochloride (GdnHCl) denaturation studies, a transition midpoint of 0.33 M was observed, corresponding to a DeltaGDH2O = 2.46 kcal. mol-1. Fluorescence studies of the sole tryptophan residue (Trp40) in apoLp-III revealed an emission lambdamax = 327 nm. Compared to free tryptophan, Stern-Volmer constants (KSV) for acrylamide and KI quenching of Trp40 fluorescence were decreased by 20-fold and sevenfold, respectively. In studies of apoLp-III-DMPC disc complexes, far-UV CD spectroscopy revealed an increase in alpha-helix content to approximately 85% and a ninefold increase in the GdnHCl-induced denaturation transition midpoint to 3 M. In studies of lipid interaction, apoLp-III was shown to disrupt both negatively charged and zwitterionic phospholipid bilayer vesicles, transforming them into discoidal complexes. Characterization of apoLp-III-DMPC discs, using 5-doxyl or 12-doxyl stearic acid as lipid-based quenching agents, revealed that Trp40 localizes near the phospholipid polar head groups. KSV values for acrylamide and KI quenching of intrinsic fluorescence of apoLp-III-DMPC discs indicate that Trp40 is embedded in the lipid milieu, with little or no accessibility to the aqueous quenchers. Given the large amount of alpha-helix in apoLp-III, the data presented support a model in which amphipathic alpha-helical segments are stabilized by helix-helix interactions and lipid association induces a protein conformational change which results in substitution of helix-helix interactions for helix-lipid contacts.
Narayanaswami
V
V
Lipid and Lipoprotein Research Group, University of Alberta, Edmonton, Canada.
Yamauchi
Y
Y
Weers
P M
PM
Maekawa
H
H
Sato
R
R
Tsuchida
K
K
Oikawa
K
K
Kay
C M
CM
Ryan
R O
RO
D000818:Animals; D001053:Apolipoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D055592:Biophysical Phenomena; D001703:Biophysics; D012831:Bombyx / Q000737:chemistry* / Q000235:genetics; D002942:Circular Dichroism; D004355:Drug Stability; D004926:Escherichia coli / Q000235:genetics; D019476:Insect Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008074:Lipoproteins / Q000378:metabolism; D008081:Liposomes; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence
D001053:Apolipoproteins; D019476:Insect Proteins; D008074:Lipoproteins; D008081:Liposomes; D011994:Recombinant Proteins; C042470:apolipophorin III