Interaction of bovine myelin basic protein with triphosphoinositide.
Despite the essential role played by myelin basic protein (MBP) in stabilizing the multilamellar structure of the myelin membrane, attempts at deciphering the structure of MPB have so far failed. Given that MBP is known to specifically interact with the membrane's lipid components, this study was designed to explore the effects of these lipids on the conformation of the protein by examining its interaction with the lipid triphosphoinositide (TPI). MBP was identified by high-performance liquid chromatography (HPLC) in myelin extracted from cow's brain and its molecular weight determined. In aqueous solution, MBP showed a random coil structure confirmed by its circular dichroism (CD) spectra. Possible structural changes to the protein induced by different proportions of TPI were also explored. The CD spectra of these mixtures indicated that this lipid fails to induce the adoption of a secondary structure by MBP. We then performed monolayer experiments to evaluate the type of interaction that occurs between MBP and TPI. First, the molecular weight of the protein sample was established to determine the state of the MBP within the monolayer by applying the equation for gases to the so-called gaseous zone of the monolayer for the conditions under which the expression holds. The similar molecular weights yielded by HPLC performed on dissolved samples and by the monolayers suggests that, as in solution, in the membrane the protein exists as a monomer. Monolayer data suggest forces of attraction between the two components and, through thermodynamic calculations, it was established that this interaction is spontaneous and the interaction is of the electrostatic type.