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Zinc ions stabilise the association of basic protein with brain myelin membranes
C Earl1, A Chantry, N Mohammad
1Department of Neurochemistry, Institute of Neurology, London, England.
Abstract:
Myelin basic protein (MBP) dissociated from brain myelin membranes when they were incubated (37 degrees 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 (less than 4 kilodaltons) and large (greater than 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 or 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 evident only in the presence of added calcium ions, when lower concentrations of Zn2+ (less than 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 solubilised in buffers containing Triton X-100/2 mM CaCl2 and subjected to gel filtration. Endogenous zinc, as indicated by a 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.
Insights
Zinc ions stabilize myelin basic protein (MBP) in brain membranes by influencing its dissociation and binding to proteolipid protein (PLP). Calcium ions promote MBP dissociation, while zinc ions inhibit it through distinct mechanisms.
Area of Science:
- Neuroscience
- Biochemistry
- Molecular Biology
Background:
- Myelin basic protein (MBP) is a key component of the myelin sheath in the central nervous system.
- The stability and integrity of myelin are crucial for proper nerve impulse conduction.
- Understanding the factors that regulate MBP association with myelin membranes is important for neurobiology.
Purpose of the Study:
- To investigate the role of zinc and calcium ions in the dissociation of MBP from brain myelin membranes.
- To elucidate the mechanisms by which zinc ions affect MBP-membrane interactions.
- To determine if endogenous zinc in myelin binds to MBP.
Main Methods:
- Incubation of brain myelin membranes with MBP under physiological conditions.
- Treatment with zinc and calcium ions to observe effects on MBP dissociation.
- Protease digestion and gel electrophoresis to analyze MBP fragments.
- Gel filtration chromatography to assess the binding of endogenous zinc to MBP and proteolipid protein (PLP).
Main Results:
- MBP dissociates from myelin membranes at physiological ionic strength, a process promoted by calcium and inhibited by zinc ions.
- Zinc ions exhibit dual inhibitory effects on MBP dissociation, with distinct characteristics regarding heat sensitivity and ion substitution.
- Endogenous zinc in myelin membranes co-elutes with MBP and PLP, suggesting a direct interaction.
Conclusions:
- Zinc ions play a significant role in stabilizing MBP association with brain myelin membranes.
- One mechanism of zinc-mediated stabilization involves promoting the binding of MBP to PLP.
- These findings provide insights into the molecular regulation of myelin structure and stability.