Hydrophobic regions in myelin proteins characterized through analysis of "hydropathic" profiles

Computer-generated "hydropathic" profiles were constructed for graphic comparison of the amino acid sequences for P2 protein, 18.5 kilodalton (kDa) myelin basic protein (BP), and myelin proteolipid protein (PLP). Profiles were also obtained for cytochrome b5, a membrane protein known to be capable of reversible association with lipid bilayers and of a size comparable to that of the myelin BPs. Analysis of the PLP sequence produced profiles generally compatible with the suggestions that PLP has three transbilayer and two bilayer intercalating segments. Profiles for P2 and 18.5 kDa BP were found to contain hydrophilic segments separated by relatively short hydrophobic regions. Whereas hydropathic indices in hydrophobic regions of P2, 18.5 kDa BP, and PLP fall in the value ranges recently reported for cores of globular proteins and intrabilayer domains of membrane proteins, hydrophobic sections of P2 and 18.5 kDa BP have hydropathic indices similar to those in the hydrophobic core (transprotein) regions of globular proteins. None of them are comparable to the region of cytochrome b5 known to anchor that protein in its membrane or to the segments of PLP sequence proposed as intrabilayer domains. This comparison suggests that neither BP has structural characteristics compatible with insertion into the hydrocarbon core of the myelin lipid bilayer, a conclusion that is consistent with a recently published study that identified the bilayer penetrating proteins of myelin with a hydrophobic probe. The above findings suggest an enhancement for some details of myelin architecture and a cautious approach to interpreting data for BP intercalation into bilayers.