The histone H1 family: specific members, specific functions?

The linker histone H1 binds to the DNA entering and exiting the nucleosomal core particle and has an important role in establishing and maintaining higher order chromatin structures. H1 forms a complex family of related proteins with distinct species, tissue and developmental specificity. In higher eukaryotes all H1 variants have the same general structure, consisting of a central conserved globular domain and less conserved N-terminal and C-terminal tails. These tails are moderately conserved among species, but differ among variants, suggesting a specific function for each H1 variant. Due to compensatory mechanisms and to the lack of proper tools, it has been very difficult to study the biological role of individual variants in chromatin-mediated processes. Our knowledge about H1 variants is indeed limited, and in vitro and in vivo observations have often been contradictory. Therefore, H1 variants were considered to be functionally redundant. However, recent knockout studies and biochemical analyses in different organisms have revealed exciting new insights into the specificity and mechanisms of actions of the H1 family members. Here, we collect and compare the available literature about H1 variants and discuss possible specific roles that challenge the concept of H1 being a mere structural component of chromatin and a general repressor of transcription.