Annett Rozek

Structure of a biologically active fragment of human serum apolipoprotein C-II in the presence of sodium dodecyl sulfate and dodecylphosphocholine.

We have studied the three-dimensional structure of a biologically active peptide of apolipoprotein C-II (apoC-II) in the presence of lipid mimetics by CD and NMR spectroscopy. This peptide, corresponding to residues 44-79 of apoC-II, has been shown to reverse the symptoms of genetic apoC-II deficiency in a human subject. A comparison of alpha-proton secondary shifts and CD spectroscopic data indicates that the structure of apoC-II(44-79) is similar in the presence of dodecylphosphocholine and sodium dodecyl sulfate. The three-dimensional structure of apoC-II(44-79) in the presence of sodium dodecyl sulfate, determined by relaxation matrix calculations, contains two amphipathic helical domains formed by residues 50-58 and 67-75, separated by a non-helical linker centered at Tyr63. The C-terminal helix is terminated by a loop formed by residues 76-79. The C-terminal helix is better defined and has a larger hydrophobic face than the N-terminal helix, which leads us to propose that the C-terminal helix together with the non-helical Ile66 constitute the primary lipid binding domain of apoC-II(44-79). Based on our structure we suggest a new mechanism of lipoprotein lipase activation in which both helices of apoC-II(44-79) remain lipid bound, while the seven-residue interhelical linker extends away from the lipid surface in order to project Tyr63 into the apoC-II binding site of lipoprotein lipase.

Sequence-specific 1H NMR resonance assignments and secondary structure of human apolipoprotein C-I in the presence of sodium dodecyl sulfate.

Apolipoprotein (apo) C-I is a 57-residue exchangeable plasma protein distributed mainly in high and very low density lipoprotein. In this report we present the nuclear magnetic resonance spectra of native apoC-I and synthetic apoC-I, containing selected 15N-labelled amino acids, in the presence of sodium dodecyl sulfate. The proton resonances of apoC-I are assigned and the secondary structure is estimated from the difference of measured alpha-proton chemical shifts to random coil values and the observed NOE interactions. According to these data apoC-I forms two helices, Val-4-Lys-30 and Leu-34-Lys-52, linked by an unstructured region Gln-31-Glu-33. The N-terminal segments of each helix, Val-4-Gly-15 and Leu-34-Met-38, appear to be more flexible than the helical core regions Asn-16-Lys-30 and Arg-39-Lys-52.