Asymmetric distribution of anionic sites on rat liver nuclear membranes

Abstract

The labelling characteristics of isolated rat liver cell nuclei was studied using polycationized ferritin as an ultrastructural probe for anionic sites. At low concentrations of the marker the nuclear surface was partly labelled eg. at sites of nuclear annuli. At high probe concentrations the entire cytoplasmic surface of the outer nuclear membrane bound ferritin particles. On the other hand, the cisternal surfaces of nuclear membranes could not be labelled although in parallel experiments Concanavalin A-ferritin bound to the cisternal surface of both nuclear membranes indicating free access of ferritin particles to the perinuclear space. The results indicate that nuclear membranes show a distinct vectorial asymmetry in respect to the presence of anionic surface sites. Nuclei have a negative surface charge on their cytoplasmic surface (eg. Bossman, 1973). Previously, this was attributed to the presence of sialic acid groups, but both cytochemical ( Virtanen and Wartiovaara, 1974) and biochemical studies ( Franke et al., 1976) have since indicated that nuclear membranes lack sialic acids. For the study of the topographic distribution of anionic surface sites in a complex membrane system such as the nuclear envelope, ultrastructural probes are needed. To this end polycationized ferritin was used in the present study as a marker to study high and low affinity anionic sites ( Danon et al., 1972) and possible differences between the surfaces of the outer and inner nuclear membranes in isolated rat liver cell nuclei.