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Expansion and apparent fluidity decrease of nuclear membranes induced by low calcium magnesium modulation of nuclear membrane lipid fluidity by the membrane associated nuclear matrix proteins


Journal of Cell Biology 79((2 PART 1)): 479-490
Expansion and apparent fluidity decrease of nuclear membranes induced by low calcium magnesium modulation of nuclear membrane lipid fluidity by the membrane associated nuclear matrix proteins
Macronuclei isolated from Tetrahymena are contracted in form (average diameter: 10.2 .mu.m) at a final Ca/Mg (3:2) concentration of 5 mM. Lowering ion concentration to 1 mM induces an expansion of the average nuclear diameter to 12.2 .mu.m. Both contracted and expanded nuclei are surrounded by a largely intact nuclear envelope as revealed by thin-section EM. 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/.mu.m2 determined by freeze-etch EM. Contracted nuclear membranes reveal particle-devoid areas (average size: 0.21 .mu.m2) on 59% of their fracture faces at the optimal growth temperature of 28.degree. C. About three-fifths of the number of these smooth areas disappear upon nuclear membrane expansion. ESR using 5-doxylstearic acid as a spin label indicates a higher lipid fluidity in contracted than in expanded nuclear membranes. A thermotropic lipid clustering occurs at .apprx. 17.degree. 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. These 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. Probably, 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.


Accession: 005419313



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