Interaction of ganglioside G_M1 and myelin basic protein studied by carbon‐13 and proton nuclear magnetic resonance spectroscopy

The interaction of the myelin basic protein (MBP) and the major endogenous ganglioside G_M1 in myelin of the central nervous system has been investigated using both 500‐MHz ¹H and 67.89 MHz ¹³C NMR. Titration of MBP by G_M1 resulted in ¹³C NMR signal shifts for the Ile and His residues of MBP at a G_M1/MBP mole ratio of one or less. The carbohydrate head group of G_M1 was also found to be perturbed. ¹H NMR results obtained in a similar manner demonstrated the perturbation of His and Phe residues. At a G_M1/MBP mole ratio of 0.5, small perturbation of Trp #116 was observed, and at mole ratios of two and beyond significant involvement of Phe residues and methylated Arg #107 was found. Met #167 was more perturbed than Met #20; hence, more extensive interaction of the lipid is occurring with the C‐terminus of the protein than with the N‐terminus. No resonances from G_M1 bound to MBP at mole ratios of up to one appeared in the spectra. However, as the G_M1/MBP mole ratio was increased to eight or greater a major conformational change of MBP was detected. An upfield shift of the G_M1 midchain methylene resonance was observed for the G_M1/MBP complex. This observation provides strong evidence that the state of G_M1 interacting with MBP is different from that of G_M1 micelles. The number of saturable G_M1 binding sites on MBP is estimated to be four. The data also favor a rapid exchange between bound G_M1 and G_M1 micelles. Interaction of MBP with the oligosaccharide derived from G_M1 was found to be weaker than with G_M1. Based on our data, a model for the interaction can be proposed: the first G_M1 molecule is bound to the protein molecule through its head group and hydrocarbon chains, followed by the formation of a G_M1/MBP complex with a concomitant conformational change of MBP as more G_M1 is added.