Compaction and particle segregation in myelin membrane arrays

Abstract

Compacted membrane arrays are formed in the [mouse] nerve myelin sheath by lowering the water activity (through evaporation or immersion in hypertonic solutions of nonelectrolytes or monovalent salts) or by binding specific cations (Ca2+, La3+ and tetracaine at concentrations above 5-10 mM). X-ray diffraction observations on intact, hydrated nerves treated to induce compaction provide a control to assess the significance of structural changes seen by freeze-fracture EM. Compaction inevitably leads to lateral segregation of particles away from the closely packed membrane arrays into contiguous normal, or slightly expanded, period arrays. In the particle-enriched layers, the E fracture face is more particle-dense than the P face, whereas no particles are found on either face in the compacted layers. Morphologically, compaction induced by the all-or-nothing, relatively irreversible action of specific cations cannot be distinguished from compaction to the same extent induced by the graded, reversible effects of nonelectrolytes. Compaction by NaCl resembles that by specific-cation binding in that the repeat period is independent of reagent concentration; but, like dehydration by nonelectrolytes, the extent of compaction is reversibly related to reagent concentration. NaCl-compacted myelin can be distinguished morphologically by a lack of the elongated border particles at the boundary between smooth and particle-enriched membrane observed for other compacting treatments. Fracture faces in compacted arrays are not always smooth, but the unusual appearances can be duplicated in purified myelin lipid multilayers subjected to similar treatments, which indicates that the particle-free membrane fracture faces are uninterrupted lipid hydrocarbon layers. Correlation of X-ray diffraction and EM observations provides a direct basis for identifying the intramembrane particles with transmembrane protein. The transmembrane protein appears to play a significant role in maintaining the normal membrane separation; swelling of the particle-enriched arrays in myelin compacted by tetracaine at low ionic strength provides information about the charge distribution on the transmembrane protein. Swelling of the compacted arrays following irreversible particle segregation shows that the interaction properties of the particle-free membranes are similar to those of pure lipid multilayers. Compaction and the consequent particle segregation in myelin results from conditions stabilizing close apposition of the lipid bilayers. Particle segregation in areas of close contact between other cell membranes may also be driven by interbilayer attractive forces.