The LINC complexes formed by outer nuclear KASH proteins and inner nuclear SUN proteins play important roles in connecting nuclei to the cytoskeleton. ANC-1, an ortholog of Nesprin-1/2, is proposed to physically tether nuclei to the actin cytoskeleton through its KASH domain at the C-terminus and the calponin homology (CH) actin-binding domains at the N-terminus.
anc-1 null mutants show severe nuclear positioning defects in the hypodermal syncytial cells in C. elegans. However, we found that the CH domains were dispensable for
hyp7 nuclear positioning and disruption of SUN/KASH interaction only caused mild nuclear positioning defects. In addition, in
anc-1 null mutants, the ER, mitochondria, and lipid-droplets were unanchored and sometimes moved freely in the cytoplasm. ANC-1 contains six tandem repeats predicted to form spectrin-like structures. Deletion of the spectrin-like region caused strong nuclear and ER anchorage defects. KASH proteins also contain a transmembrane domain adjacent to the luminal KASH domain, we found that the deletion of the transmembrane domain enhanced the nuclear or the ER anchorage defects of the KASH deletion mutant. In the
hyp7, ANC-1 had a similar distribution pattern as the ER, and deletion of the tandem repeats strongly affected ANC-1's localization. Here, we propose a cytoplasmic integrity model where ANC-1 functions in positioning nuclei, ER, mitochondria, and likely other organelles in place, probably through the spectrin-like region.