Repeated helical pattern in apolipoprotein-A-I

Abstract

THE binding of lipids into globules by the serum lipid-binding proteins depends on the unique structures of these proteins. Many have a high α helix content^1–4 which is enhanced by the binding of lipid, and their amino acid sequences^5–8 suggest that the helices are often ‘amphipathic’ with a long hydrophobic face buried in the lipid surface^9,10 and an exposed polar face which interacts specifically with the charged head groups of phospholipids such as lecithin. Apolipoprotein-A-I (or -Gln-I) is the larger of the two major human high density plasma lipoproteins^11–13, with 245 amino acids and a helix content³ of about 70% in the native complex. Physical measurements^10,14–16 and a study of the sequence^10,11 suggest that in several of its complexes reconstituted from phospholipids, the protein has a loosely folded structure of between 9 and 13 helices which float in the surface of a disk-shaped bilayer 5.5 nm thick and 11 nm in diameter^17,18, rather as logs float on water. With pure lecithin and A-I protein, each complex contains two protein and about 200 lipid molecules¹⁹. The pattern of non-polar groups in A-I protein is unusually regular, even for a helical structure, and this suggested that the sequence may have evolved by internal gene duplication²⁰ in the same way as some other long repetitive structures^21,22. Barker and Dayhorf²³, Fitch²⁴ and I independently found a regular 11- or 22-residue repeat pattern in it. Here I report an analysis of the repeats and discuss their structural significance.