Abstract
Lamins are intermediate filament proteins and the major component of the nuclear lamina. Current views of the lamina are based on the remarkably regular arrangement of lamin LIII in amphibian oocyte nuclei. We have re-examined the LIII lamina and propose a new interpretation of its organization. Rather than consisting of two perpendicular arrays of parallel filaments, we suggest that the oocyte lamina consists of parallel filaments that are interconnected in register to give the impression of a second set of perpendicular filaments. We have also used the oocyte system to investigate the organization of somatic lamins. Currently, it is not feasible to examine the organization of somatic lamins in situ because of their tight association with chromatin. It is also difficult to assemble vertebrate lamin filaments in vitro. Therefore, we have used the oocyte system, where exogenously expressed somatic B-type and A-type lamins assemble into filaments. Expression of B-type lamins induces the formation of intranuclear membranes that are covered by single filament layers. LIII filaments appear identical to the endogenous lamina, whereas lamin B2 assembles into filaments that are organized less precisely. Lamin A induces sheets of thicker filaments on the endogenous lamina and significantly increases the rigidity of the nuclear envelope.
Highlights
The nuclear lamina is a protein meshwork located between the inner nuclear membrane and the peripheral chromatin of metazoan cells
Rather than consisting of two perpendicular arrays of parallel filaments, we suggest that the oocyte lamina consists of parallel filaments that are interconnected in register to give the impression of a second set of perpendicular filaments
We show that applying high-resolution field-emission scanning electron microscopy to the amphibian oocyte expression system is a unique means to analyse the organization of lamin filaments and their assembly into higher-order structures
Summary
The nuclear lamina is a protein meshwork located between the inner nuclear membrane and the peripheral chromatin of metazoan cells. Targeting of lamins to the inner nuclear membrane depends on the presence of a nuclear localization signal (NLS) and posttranslational lipidation (Mattout et al, 2006; Nigg et al, 1992; Young et al, 2005). Lamins interact with both integral proteins of the inner nuclear membrane and with chromatin-binding proteins (Burke and Stewart, 2002; Gruenbaum et al, 2003). The importance of lamin function is highlighted by targeted disruption of the lamins, by genetic analysis and RNAi, as well as by mutations in genes encoding lamins that cause a wide range of heritable human diseases – the laminopathies (Broers et al, 2006; Gruenbaum et al, 2003; Mattout et al, 2006)
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