A novel linker histone-like protein is associated with cytoplasmic filaments in Caenorhabditis elegans

The histone Ell complement of Caenorhabditis elegans contains a single unusual protein, H1.X. Although H1.X possesses the globular domain and the canonical three-domain structure of linker histones, the amino acid composition of H1.X is distinctly different from conventional linker histones in both terminal domains. We have characterized H1.X in C elegans by antibody labeling, green fluorescent protein fusion protein expression and RNA interference. Unlike normal linker histones, H1.X is a cytoplasmic as well as a nuclear protein and is not associated with chromosomes. H1.X is most prominently expressed in the marginal cells of the pharynx and is associated with a peculiar cytoplasmic cytoskeletal structure therein, the tonofilaments. Additionally H1.X::GFP is expressed in the cytoplasm of body and vulva muscle cells, neurons, excretory cells and in the nucleoli of embryonic blastomeres; and adult gut cells. RNA interference with H1.X results in uncoordinated and egg laying defective animals, as well as in a longitudinally enlarged pharynx. These phenotypes indicate a cytoplasmic role of H1.X in muscle growth and muscle function.