Nuclear titin interacts with A- and B-type lamins in vitro and in vivo

Lamins form structural filaments in the nucleus. Mutations in A-type lamins cause muscular dystrophy, cardiomyopathy and other diseases, including progeroid syndromes. To identify new binding partners for lamin A, we carried out a two-hybrid screen with a human skeletal-muscle cDNA library, using the Ig-fold domain of lamin A as bait. The C-terminal region of titin was recovered twice. Previous investigators showed that nuclear isoforms of titin are essential for chromosome condensation during mitosis. Our titin fragment, which includes two regions unique to titin (M-is6 and M-is7), bound directly to both A- and B-type lamins in vitro. Titin binding to disease-causing lamin A mutants R527P and R482Q was reduced 50%. Studies in living cells suggested lamin-titin interactions were physiologically relevant. In Caenorhabditis elegans embryos, two independent C. elegans (Ce)-titin antibodies colocalized with Ce-lamin at the nuclear envelope. In lamin-downregulated [lmn-1(RNAi)] embryos, Ce-titin was undetectable at the nuclear envelope suggesting its localization or stability requires Ce-lamin. In human cells (HeLa), antibodies against the titin-specific domain M-is6 gave both diffuse and punctate intranuclear staining by indirect immunofluorescence, and recognized at least three bands larger than 1 MDa in immunoblots of isolated HeLa nuclei. In HeLa cells that transiently overexpressed a lamin-binding fragment of titin, nuclei became grossly misshapen and herniated at sites lacking lamin B. We conclude that the C-terminus of nuclear titin binds lamins in vivo and might contribute to nuclear organization during interphase.