SynopsisThe conformation of (3-casein A in the monomeric and thermally aggregated states has been investigated by a range of techniques. P-Casein exists as a monomer in solution a t 4°C and a t concentrations up to a t least 3 g/dl. The molecule is flexible and exhibits a lot of segmental motion, but its secondary structure is not wholly random coil; about one-third of the polypeptide chain is ordered and the likely locations of these regions are discussed. The radius of gyration, representing the time-average distribution of the flexible chain, is 46 A. Increasing temperature leads to aggregation of the P-casein molecules. The degree of association is very sensitive to experimental conditions, and under our conditions a 14-mer exists a t 20°C. The aggregate is spherical with a radius of about 100 A. The interior of the aggregate is relatively disordered, and the P-casein molecules remain in a largely flexible, hydrated conformation. The volume restriction of the protein molecules which occurs on association leads tvsome immobilization of the hydrophobic C-terminal region, which is packed toward the center of the aggregate.

The detailed molecular structure of the complex formed by the apoprotein from porcine high density lipoprotein and dimyristoyl phosphatidylcholine (lecithin) has been investigated by a range of physical techniques. The complex, an oblate ellipsoid with major axis 11.0 nm and minor axis 5.5 nm (see the accompanying paper), is comprised of a section of lecithin bilayer with apoprotein at the surface. The main site of interaction between protein and lipid is in the lipid glycerophosphorylcholine group region; as with native high density lipoprotein the surface of the particle consists of a mosaic of lecithin polar groups and protein. The formation of this mosaic reduces the cooperativity of the lecithin chain motions and changes the curvature of the lipid-water interface, as compared to a bilayer. Otherwise, there are no major changes in lecithin motions indicating that no strong binding of lipid to protein occurs. The interaction involves the intercalation of amphipathic, 60 :d a-helical, apoprotein molecules among the lecithin molecules so that the protein resides at the lipid-water interface. The apoprotein has a high affinity for the lipid-water interface but specific lipid-protein interactions are not involved.Although proteins which are associated with lipids in vivo tend to be more hydrophobic than highly water-soluble proteins [ 1,2], high hydrophobicity alone is insufficient to allow complex-formation between lipid and protein in solution [3]. It has been pointed out before that there are more specific requirements for the protein, and it is the purpose of this study [3 -6a] to elucidate these for a particular class of lipid-associated proteins, namely the apoproteins of serum high density lipoproteins.Earlier work has established the chemical structure [4,7] and morphology [8,9] of high density lipoprotein (density 1.07-1.21 g/ml); leading references to earlier work are cited in these papers and [6a]. The two major apoproteins isolated [lo, 111 from human serum high density lipoproteins have been designated [32] as apo A-I and apo A-11. The amino acid sequences of these two apolipoproteins from human serum are known [13,14]. The major peptides (apo A-I) from human and porcine apoproteins have been compared [4,15,16] and shown to have the same general properties. The ability of the total apoproteins to recombine with the total lipids of high density lipoprotein to reconstitute particles similar to the original ~_ _ _ Abhreviations. NMR, nuclear magnetic resonance; ESR, electron spin resonance; CD, circular dichroism. high density lipoprotein has been well documented [ll]. Also the interaction with lecithin alone to give lipid-protein complexes which can be isolated by density-gradient ultracentrifugation has been described [5,18] and shown to be particularly amenable to study by physico-chemical techniques [19 -241. The results presented here and in [6a] are part of a detailed study of the complex formed by the total apoprotein fraction derived froin porcine high density lipoprotein and dimyristoyl lecit...