Conformation of brain proteolipid apoprotein. Effects of sonication and n-octyl-beta-D-glucopyranoside detergent.

The conformation of brain proteolipid apoprotein (PLA) has been investigated using infrared spectroscopy and freeze-fracture electron microscopy. For this purpose, spectroscopic samples consisting of a mixture of liquid paraffin and wet protein have been prepared. These systems have allowed us to record the infrared spectra of PLA at neutral pH. The amide I and III regions reveal the existence of a predominantly alpha-helical structure, as well as the presence of minor beta-strands and random coil forms. The effect of sonication and a non-denaturing detergent, (n-octyl-beta-D-glucopyranoside), on the structure of the protein have also been investigated. Sonication produces an increase of the beta and unordered structures at the expense of the alpha-helical conformation. These structural changes are enhanced in the presence of the non-ionic detergent n-octyl-beta-D-glucopyranoside. Lipids protect the native protein structure from the effects of sonication. The aforementioned detergent changes the PLA conformation by increasing the alpha-helical content at the expense of beta-sheet and random coil forms. Therefore the PLA structure seems to be similar to the structures of other proteins intrinsic to non-neural membranes. The effects investigated also suggest that PLA behaves in a conformationally flexible manner.