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.