LINC complexes do not maintain the space between nuclear membranes, except in cells exposed to mechanical stress, Cain et al. reveal.
SUN proteins in the inner nuclear membrane and KASH proteins in the outer nuclear membrane form LINC complexes that regulate multiple processes by connecting the nucleoskeleton to the cytoskeleton. LINC complexes are also thought to maintain a uniform gap of ∼50 nm between the outer and inner nuclear membranes. This perinuclear space widens in HeLa cells depleted of SUN proteins, but the role of LINC complexes in maintaining nuclear envelope architecture in vivo hasn’t been tested.
Cain et al. examined the nuclear envelopes of C. elegans larvae lacking the only somatic SUN protein, UNC-84. Surprisingly, the absence of UNC-84 had no effect on nuclear envelope spacing in most worm tissues. In the body wall muscle cells of unc-84 mutants, however, the nuclear membranes became widely separated in regions predicted to undergo mechanical strain. Cain et al. therefore think that LINC complexes are only required to hold nuclear membranes together in cells subjected to high mechanical stress. Consistent with this, HeLa cells experience intracellular tension when plated onto tissue culture dishes.
Cain et al. wondered whether the size of LINC complexes nevertheless sets the width of the perinuclear space. An UNC-84 mutant lacking most of its luminal domain formed functional LINC complexes, and, even though these complexes are predicted to be much shorter, the nuclear membranes of worms expressing this mutant were still spaced 50 nm apart. Senior author Daniel Starr thinks that inherent properties of the nuclear membranes (and the contiguous ER) might instead determine nuclear envelope spacing.
Text by Ben Short