Nanosecond rotational motions of apolipoprotein C-I in solution and in complexes with dimyristoylphosphatidylcholine

Abstract

Human apolipoprotein C-I (apo C-I) in solution, in monomeric and oligomeric form, and in micellar complexes with dimyristoylphosphatidylcholine (DMPC), below and above the phase transition temperature of DMPC, was investigated with steady-state and time-resolved fluorescence methods. The environment of the Trp residue of apo C-I, in each physical state, was evaluated from fluorescence spectra and their changes upon KI quenching. Rotational correlation times of Trp residues were obtained from fluorescence anisotropy decay measurements. Static fluorescence anisotropy was determined as a function of temperature for the Trp residues of apo C-I in all physical states and for diphenylhexatriene dissolved in apo C-I X DMPC complexes. It was found that the Trp residues of apo C-I in solution are exposed from 75 to 88% to the aqueous medium, depending on the state of self-association. On the other hand, the Trp residues in apo C-I X DMPC complexes are only 42-45% exposed to KI quenching through an environment distinct from water. Apolipoprotein C-I in all its physical forms had two rotational correlation times associated with Trp motions: a longer one dependent on the size and flexibility of the entire particle and a very short one in the range from 0.2 to 0.4 ns. The later correlation times correspond to local Trp residue motions. These Trp motions were not significantly affected by a transition from the gel to the liquid-crystalline state of the lipid in apo C-I X DMPC complexes, suggesting that there is no coupling between the local motions of lipids and those of Trp side chains of apo C-I.

α-Synuclein structures from fluorescence energy-transfer kinetics: Implications for the role of the protein in Parkinson's disease

“…In comparison to other lipid-binding proteins, the micelleinduced changes in ␣-syn Trp anisotropy decay are rather modest. Melittin binding to liposomes, for example, produces a larger r o value (0.185) and longer rotational correlation time (10 ns) (59); Trp rotation in human apolipoprotein C-I slows dramatically (44 ns) in the presence of micelles (60). Although the blue shift of the Trp fluorescence indicates that the indole is in a strongly hydrophobic micellar environment, the rapid anisotropy decay reveals that the side chain retains a considerable degree of rotational freedom.…”

Resolution of the fluorescence decay of the two tryptophan residues of lac repressor using single tryptophan mutants

“…The observed biexponential decay has been interpreted as arising from the intrinsic monoexponential decay of the two individual tryptophan residues. However, examination of many single-tryptophan-containing proteins, reveals that the fluorescence decay of individual residues is not monoexponential (8,(10)(11)(12)(13)(14)(15)(16)(17)(18). These results have been interpreted as being due to the intrinsic heterogeneous decay of tryptophan, interaction of the tryptophan residue with charged amino-acid groups of the nearby protein matrix, and conformational heterogeneity of the proteins in solution.…”

Time‐Dependent Fluorescence Intensity and Depolarization of Diphenlhexatriene in Micellar Complexes of Apolipoprotein C‐I and Dimyristoylglycerophosphocholine

“…The resulting electron micrographs showed characteristic stacks of discoidal particles essentially identical to those previously described by us [16,17] and others [2,18] for complexes of apolipoproteins with phosphatidylcholines. Detailed characterizations of such micellar complexes of apolipoprotein A-I, A-11, and C with Myr,Gro-P-Cho are thoroughly documented in the literature [I, 2,4,6,8,16,19,20]. The complexes of apoC-I with Myr,Gro-P-Cho were stable for at least 3 months when stored at 4°C.…”

“…tophanyl residues, which are at least partially exposed to the lipid, do not sense the phase transition from gel to liquidcrystalline state of the bulk lipid [8]. Here we examine the timedependent fluorescence intensity and anisotropy of Ph,C,H,, dissolved in the lipid domains of the same complexes, with the objective of determining the effects of apoC-I on the bulk lipid behavior, in terms of rotational motions and ordering of the probe.…”