The interaction of human apolipoprotein C-I with sub-micellar phospholipid

Mature human apolipoprotein C-I (apoC-I), comprising 57 amino acids, is the smallest member of the plasma apolipoprotein family. Amphipathic helical r egions within apoC-I, common to this class of proteins, are mediators of li pid binding, a process that underlies the functional properties of apoC-I, including the capacity to activate the plasma enzyme LCAT, to disrupt apoE mediated receptor interactions and to inhibit cholesterol ester transfer pr otein. To examine apoC-I/phospholipid interactions, we have developed an ex pression system in Escherichia coli to obtain purified apoC-I with yields o f approximately 4-5 mg per L of culture. The purified product has propertie s similar to plasma-derived apoC-I including self-association in the lipid- free state and induced alpha -helical content in the presence of egg-yolk p hosphatidylcholine and dimyristoylglycerophosphocholine vesicles. We chose the short-chain phospholipid, dihexanoylglycerophosphocholine (Hex(2)Gro-PC ho), to examine the interaction of apoC-I with submicellar phospholipid. Ci rcular dichroism spectroscopy and cross-linking experiments show that apoC- I acquires helical content and remains self-associated at submicellar conce ntrations of Hex(2)Gro-PCho (4 mm). Sedimentation equilibrium studies of ap oC-I at submicellar levels of Hex(2)Gro-PCho and analysis of the effects of apoC-I on the H-1 NMR spectrum of Hex(2)Gro-PCho indicate micelle inductio n by apoC-I, and establish the capacity of apoC-I to assemble individual ph ospholipid molecules.