chromo shadow domain of insect heterochromatin protein 1a
The chromo shadow domain (CSD) is always found in association with a related N-terminal chromo (CHRromatin Organization MOdifier) domain. CSD domains have only been found in proteins that also possess a chromodomain, while chromodomains can exist in isolation. HP1 has two conserved protein-protein interaction domains, a single N-terminal chromodomain (CD) which can bind to histone proteins via methylated lysine residues, and a related C-terminal CSD which is responsible for the homodimerization and interaction with a number of chromatin-associated non-histone proteins; a flexible hinge region separates the CD and CSD and may bind nucleic acid. HP1 is a highly conserved non-histone chromosomal protein that is evolutionarily conserved from fission yeast to plants and animals. The HP1 CSD, in addition to interacting with various proteins bearing the PXVXL motif, also interacts with a region of histone H3 that bears the similar PXXVXL motif. There are three human homologs of HP1 proteins: HP1alpha (also known as Cbx5), HP1beta (also known as Cbx1), and HP1gamma (also known as Cbx3). The CSD domains of all three human HP1 homologs have similar affinities to the PXXVXL motif of histone H3. In Drosophila, there are at least five HP1 family proteins, this subgroup includes the CSD of Drosophila melanogaster HP1a.
Jiyao W et al.(2023). "The conserved domain database in 2023.",