HP1 proteins are fundamental units of heterochromatin packaging that are enriched at the centromeres and telomeres of nearly all eukaryotic chromosomes with the notable exception of budding yeast, in which a yeast-specific silencing complex of SIR (silent information regulatory) proteins serve a similar function.
Members of the HP1 family are characterized by an N-terminal chromodomain and a C-terminal chromoshadow domain, separated by a hinge region.
HP1 was originally discovered by Tharappel C James and Sarah Elgin in 1986 as a factor in the phenomenon known as position effect variegation in Drosophila melanogaster.
In Arabidopsis thaliana (a plant), there is one structural homolog: Like Heterochromatin Protein 1 (LHP1), also known as Terminal Flower 2 (TFL2).
[4] HP1β interacts with the histone methyltransferase (HMTase) Suv(3-9)h1 and is a component of both pericentric and telomeric heterochromatin.
[14] Nevertheless, favorable interactions between nearest neighbors of HP1 lead to limited spreading of HP1 and its marks along the nucleosome chain in vivo.
In this pathway HP1 acts as a mediator protein for repression of alternative splicing of the EDA exon.