Zinc Ions Stabilise the Association of Basic Protein with Brain Myelin Membranes

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Abstract
Myelin basic protein (MBP) dissociated from brain myelin membranes when they were incubated (37.degree. C; pH 7.4) at physiological ionic strength. Zinc ions inhibited, and calcium promoted, this process. Protease activity in the membrane preparations cleaved the dissociated MBP into both small (8 kilodaltons)fragments. The latter were detected, together with intact MBP, by gel electrophoresis of incubation media. Zinc ions appeared to act in two distinct processes. In the presence of absence of added CaCl2, zinc ions in the range 0.1-1 mM inhibited MBP-membrane dissociation. This process was relatively insensitive to heat and Zn2+ could be substituted by either copper (II) or cobalt (II) ions. A second effect was evidence only in the presence of added calcium ions, when lower concentrations of Zn2+ (<0.1 mM) inhibited MBP-membrane dissociation and the accumulation of intact MBP in incubation media. This process was heat sensitive and only copper (II), but not cobalt (II), ions could replace Zn2+. To determine whether endogenous zinc in myelin membranes is bound to MBP, preparations were solubilized in buffers containing Triton X-100/2 mM CaCl2 and subjected to gel filtration. Endogenous zinc, as indicated by dithizone-binding method, eluted with fractions containing both MBP and proteolipid protein (PLP) Thus, one means whereby zinc stabilises association of MBP with brain myelin membranes may be by promoting its binding to PLP.