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?

Abstract

Macronuclei isolated from Tetrahymena are contracted in form (average diameter:10.2 /.tm) at a final Ca/Mg (3:2)concentration of 5 raM. Lowering the ion concentration to 1 mM induces an expansion of the average nuclear diameter to 12.2/xm. 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/ /zm ~ determined by freeze-etch electron microscopy. Contracted nuclear membranes reveal particle-devoid areas (average size: 0.21 /xm 2) on 59% of their fracture faces at the optimal growth temperature of 28~ 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 expanded nuclear membranes. Moreover, a thermotropic lipid clustering occurs at -17~ 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. KEY WORDS nuclear envelope nuclear membranes nuclear matrix membrane lipid fluidity isothermic membrane lipid segregation Biomembranes are widely considered as being 'fluid' entities, i.e., membrane integral proteins can diffuse laterally and/or rotationally within a fluid lipid bilayer continuum (41). Recently, evidence has accumulated that the mobility of membrane integral proteins can be controlled, inter alia, by membrane-associated skeletal proteins (e.g., references 11, 37, 42, 53).J. CELL BIOLOGY ~ The Rockefeller University Press 9

In vitro nuclear transport of ribosomal ribonucleoprotein: temperature affects quantity but not quality of exported particles.

“…Finally, it is noteworthy that nuclei isolated from Tetrahymena are structurally intact at the transport level, i.e., they are surrounded by a well-preserved nuclear envelope still revealing typical in situ properties under export conditions (11,40; see also 12). Therefore, it is an attractive speculation that the nuclear envelope represents the temperature-dependent barrier for the export of rRNP particles that are "free" in nuclei.…”

“…After the cells were disrupted with glycerol (11,40), nuclei were isolated and purified on two-step sucrose gradients by our previous method (41) with the modification described by Herlan et al (15).…”

Thermal down‐regulation of exportable rRNA in nuclei

“…The cells were disintegrated using our previous glycerol technique (Wunderlich et al, 1978;Giese et al, 1979). The nuclei were then isolated and purified according to our previous method (Wunderlich and Herlan, 1977) with the modification described by Herlan et al (1980).…”

“…We previously developed such a cell-free system of rRNA export in the unicellular eukaryote Tetruhymenu (Herlan et al, 1980). This system operates with nuclei that are surrounded by a structurally intact nuclear envelope (Wunderlich et al, 1978;Giese et al, 1979; cf. also and can export precursor ribosomal ribonucleoprotein (rRNP) particles (Herlan et al, 1980).…”

Isolation and Characterization of Bovine Lymphocyte Nuclear Matrix

“…The nuclear matrix appears to be a ubiquitous structure that is present in almost all eukaryotic cells. Much evidence is accumulating to suggest that the nuclear matrix not only may function as the skeleton of the nucleus but also may be associated with many dynamic aspects of nuclear function: nuclear swelling (30,31), DNA replication (5,7,13,16,27), RNA transcription (1,11,23), hormone binding (3), T-antigen binding (9), processing and transport of RNA (22,29), etc.…”