Histones, key components of eukaryotic chromatin, come in two types: core histones and linker histones. Core histones are part of the nucleosome and essential in all eukaryotes. Linker histones, by contrast, bind to the linker DNA between nucleosomes and are dispensable in some organisms. Nonetheless they are exclusively nuclear and function in chromatin organization —right? Well, not exactly. Ekkehard Schulze and co-workers have identified a novel linker histone in nematodes, H1. X, that resides in the cytoplasm (see). Sequence analysis reveals that H1.X is a bona fide member of the linker histone family, but the authors find that it predominantly localizes not to chromatin but to tonofilaments — specialized bundles of intermediate filaments in marginal cells of the pharynx. The protein is also present in the cytoplasm of body-wall and vulval muscle cells. Nematodes in which Schulze and co-workers reduced H1.X levels by RNAi exhibit uncoordinate movement and defective egg laying. The authors therefore propose that this unusual histone has a cytoskeletal role that is important for muscle function and development.
