Interaction of lipid-bound myelin basic protein with actin filaments and calmodulin.

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocytes (OLs) and is believed to be responsible for adhesion of these surfaces in the multilayered myelin sheath. MBP in solution has been shown by others to bind to both G- and F-actin, to bundle F-actin filaments, and to induce polymerization of G-actin. Here we show that MBP bound to acidic lipids can also bind to both G- and F-actin and cause their sedimentation together with MBP-lipid vesicles. Thus it can simultaneously utilize some of its basic residues to bind to the lipid bilayer and some to bind to actin. The amount of actin bound to the MBP-lipid vesicles decreased with increasing net negative surface charge of the lipid vesicles. It was also less for vesicles containing the lipid composition predicted for the cytosolic surface of myelin than for PC vesicles containing a similar amount of an acidic lipid. Calmodulin caused dissociation of actin from MBP and of the MBP-actin complex from the vesicles. However, it did not cause dissociation of bundles of actin filaments once these had formed as long as some MBP was still present. These results suggest that MBP could be a membrane actin-binding protein in OLs/myelin and its actin binding can be regulated by calmodulin and by the lipid composition of the membrane. Actin binding to MBP decreased the labeling of MBP by the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[(125)I]iodophenyl)diazirine (TID), indicating that it decreased the hydrophobic interactions of MBP with the bilayer. This change in interaction of MBP with the bilayer could then create a cytosol to membrane signal caused by changes in interaction of the cytoskeleton with the membrane.