INTERACTIONS OF MYELIN BASIC-PROTEIN WITH PALMITOYLLYSOPHOSPHATIDYLCHOLINE - CHARACTERIZATION OF THE COMPLEXES AND CONFORMATIONS OF THE PROTEIN

The stoichiometry of palmitoyllysophosphatidylcholine/myelin basic pro tein (PLPC/MBP) complexes, the location of the protein in the lysolipi d micelles, and the conformational changes occurring in the basic prot ein and peptides derived from it upon interaction with lysolecithin mi celles were investigated by circular dichroic spectropolarimetry, ultr acentrifugation, electron para magnetic resonance (EPR) and P-31, C-13 , and H-1 nuclear magnetic resonance spectroscopy (NMR), and electron microscopy. Ultracentrifugation measurements indicated that well-defin ed complexes were formed by the association of one protein molecule wi th approximately 141 ly solipid molecules. Small-angle X-ray scatterin g data indicated that the PLPC/MBP complexes form particles with a rad ius of gyration of 3.8 nm. EPR spectral parameters of the spin labels 5-, and 16-doxylstearate incorporated into lysolecithin/basic protein aggregates, and C-13- and H-1-NMR relaxation times of PLPC indicated t hat the addition of the protein did not affect the environment and loc ation of the labels and the organization of the lysolipid micelles. Th e data suggested that MBP lies primarily near the surface of the micel les, with segments penetrating beyond the interfacial region into the hydrophobic interior, but without any part of the protein being protec ted against rapid exchange of its amide groups with the aqueous enviro nment. The basic protein acquired about 20% alpha-helix when bound to lysolipid micelles. Circular dichroic spectra of sequential peptides d erived by cleavage of the protein revealed the formation of alpha-heli cal regions in the association with lysolecithin. Specific residues in myelin basic protein that participated in binding to the micelles wer e identified from magnetic resonance data on changes in the chemical s hifts and intensities of assigned resonances, and line broadening of p eaks by fatty acid spin-labels incorporated into the micelles.