Lamin A is part of the internal nucleoskeleton of human erythroleukemia cells

Nuclear lamins are the most abundant components of the nuclear lamina, a 10 -50-nm-thick fibrous layer underlying the inner nuclear envelope membrane. Nevertheless, a number of recent investigations performed on epithelial and fibroblast cells have suggested that nuclear lamins are also present withi n the nucleoplasm and could be important constituents of the nucleoskeleton . We have studied the subnuclear distribution of lamins A and B1 in human e rythroleukemia cells by using immunoblotting analysis and immunofluorescent staining of fractionated nuclei. In intact cells and isolated nuclei, anti bodies to lamins A and B1 mainly stained the nuclear periphery, although so me immunoreactivity was detected in the nuclear interior. However, when chr omatin was removed by nuclease digestion and extraction with nonionic deter gent or solutions of high ionic strength, a previously masked immunoreactiv ity for lamin A, but not for lamin B1, became evident in the internal part of the residual structures representing the nuclear matrix or scaffold. Pre ferential localization of lamin A to the inner part of the nucleus was also demonstrated by the presence of the majority of iamin A in the solubilized inner nuclear network subfraction. In contrast, lamin B1 was mainly recove red in the fraction corresponding to the nuclear periphery. Double labeling experiments showed that lamin A, but not lamin B1, colocalized with coiled and GATA-1 bodies. Thus, our results support the hypothesis that lamin A, but not lamin B1, may be a component of an internal nucleoskeleton in human erythroleukemia cells. J. Cell. Physiol. 178:284-295, 1999. (C) 1999 Wiley -Liss, Inc.