The Journal of Cell Biology, Vol 79, 479-490, Copyright © 1978 by The Rockefeller University Press
Expansion and apparent fluidity decrease of nuclear membranes induced by low Ca/Mg. Modulation of nuclear membrane lipid fluidity by the membrane-associated nuclear matrix proteins?
F Wunderlich, G Giese and C Bucherer
Macronuclei isolated from Tetrahymena are contracted in form (average
diameter: 10.2 micron) at a final Ca/Mg (3:2)concentration of 5 mM.
Lowering the ion concentration to 1 mM induces an expansion of the average
nuclear diameter to 12.2 micron. Both contracted and expanded nuclei are
surrounded by a largely intact nuclear envelope as revealed by
thin-sectioning electron microscopy. Nuclear swelling is accompanied by an
expansion of the nuclear envelope as indicated by the decrease in the
frequency of nuclear pore complexes from 52.6 to 42.1 pores/micron2
determined by freeze-etch electron microscopy. Contracted nuclear membranes
reveal particle-devoid areas (average size: 0.21 micron2) on 59% of their
fracture faces at the optimal growth temperature of 28 degrees C. About
three-fifths of the number of these smooth areas disappear upon nuclear
membrane expansion. Electron spin resonance using 5-doxylstearic acid as a
spin label indicates a higher lipid fluidity in contracted than in expa,ded
nuclear membranes. Moreover, a thermotropic lipid clustering occurs at
approximately 17 degrees C only in expanded nuclear membranes. In contrast
to the nuclear membrane- bound lipids, free lipids extracted from the
nuclei rigidify with increasing Ca/Mg concentrations. Our findings are
compatible with the view that the peripheral layer of the fundamental
nuclear protein- framework, the so-called nuclear matrix, can modulate,
inter alia, the lipid distribution and fluidity, respectively, in nuclear
membranes. We suggest that a contraction of the nuclear matrix's peripheral
layer induces a contraction of the nuclear membranes which, in turn, leads
to an isothermic lateral lipid segregation within nuclear membranes.
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