The thermodynamically stable state of myelin basic protein in aqueous solution is a flexible coil

Conformational studies of myelin basic protein (MBP) in solution generally have used protein purified in organic solvents and acid. The use of such conditions raises the possibility that the secondary structure reported for the basic protein represents a denatured state. Therefore we have purified this protein by using a procedure that avoids denaturants. Bovine myelin was extracted with 0.2 M-CaCl2 and the protein was purified from the supernatant by chromatography on Sephadex G-75. The conformation of the basic protein was characterized by using c.d. and 1H-n.m.r. spectroscopy. In solution, it appeared to be predominantly randomly coiled, with only small segments of persistent structure. However, in the presence of myristoyl lysophosphatidylcholine the secondary structure of MBP became more ordered, and sedimentation-velocity experiments showed that MBP aggregated. Comparison of our results with published data indicates that Ca2+-extracted basic protein behaves similarly to the protein purified by traditional methods with respect to its ordered conformation in solution in the absence and in the presence of lipid and with respect to its self-association. Thus its thermodynamically stable structure in aqueous solution appears to be a highly flexible coil.