Conformational studies of a consensus sequence peptide (CSP) and a real sequence peptide (RSP) of apolipoproteins by circular dichroism spectroscopy and x-ray crystallography
The apolipoproteins are essential for the structure of the plasma lipoproteins and their functions in lipid transport, as cofactors for enzymes or ligands for receptors important in lipid metabolism. The sequences of the apolipoproteins contain 22-residue tandem repeats thought to form amphipathic α-helices that interact with lipids. Two peptide analogs of apolipoproteins, a consensus sequence peptide (CSP) and a real sequence peptide (RSP), were synthesized. Both peptides contain two repeats (44 residues) that are thought to form two 20 residue α-helices connected by a loop.
The sequence of CSP ((PLAEELRARLRAQLEELRERLG)2) was based on the consensus sequence of apolipoproteins and represents an idealized model for the fundamental structural motif. Circular dichroism (CD) showed that CSP is 87% α-helical. The helicity increased in the presence of 30% 2,2,2-trifluoroethanol (TFE) which induces secondary structure formation, and in 0.6% n-octyl, β-glucopyranoside (BOG), a nonionic detergent. RSP from apoA-I, residues 99-142, (PYLDDFQKKWQEEMELYRQKVEPLRAELQEGARQKLHELQEKLS) showed less α-helicity (∼30%). However, the a-helicity increased to ∼72% in 30% TFE and to ∼54% in 0.6% BOG at 25°C. In contrast to CSP and apoA-I, but similar to the small apoproteins A-II and C-I, RSP undergoes low-temperature unfolding in BOG above its critical micellar concentration (CMC). The unfolding is the result of micelle dissociation caused by an increase in the CMC of BOG with decreasing temperature. This indicates that only BOG micelles bind to RSP and thereby increase the α-helicity of RSP and model the lipid environment of a lipoprotein.
Single crystals of CSP were grown from 17% polyetheleneglycol, 25% 2-propanol, 0.1M Na citrate, pH7.I by vapor diffusion. X-ray diffraction data to 1.76Å resolution indicate the space group P212121 (a = 24.6Å, b = 33.7Å, c = 45.9Å) with one molecule per asymmetric unit. The diffraction pattern was dominated by 10Å reflections on the h0l plane, and strong diffraction on the 5.4Å layer line. Thus, the α-helices are parallel to the b-axis and the helical length is ∼30Å (20 residues), substantiating a helix-loop-helix structure. Molecular replacement and multiple-wavelength anomalous dispersion methods were used to determine the diffraction phases for structure solution and refinement. The results showed that the CSP molecules in the crystal form parallel four-helix bundle.