Effect of microheterogeneity on the structure and function of the myelin basic protein

The basic protein is a major component of central nerve myelin containing species with varying amounts of phosphorylation and deamidation in situ. Components of basic protein were separated on the basis of net charge. Differences in ionic interactions of components of basic protein in a two-phase partition system with beef brain phosphatidyl serine were small, but there was a tendency for less charged components to require less lipid to achieve the same degree of solubilization into the chloroform phase. Spectroscopic properties of the components were identical using the following techniques: far ultraviolet circular dichroism, difference ultraviolet spectroscopy, fluorescent determination of tyrosine pK values and fluorescence energy transfer. At alkaline pH transfer from Trp to Tyr O- was 25% efficient which indicates that the closest accepting tyrosine is approx. 10 A away (the closest fluorescent tyrosine is over 20 A away). In 6 M guanidinium chloride this transfer was abolished while difference spectra indicated that the aromatic amino acids became more exposed in this solvent. These results show that the basic protein has definite secondary structure but that is not affected by the post-translation modifications.