chromo shadow domain of heterochromatin protein 1 gamma homolog gamma
Chromo shadow domain (CSD) of heterochromatin protein 1 gamma homolog gamma (also known as HP1gamma, Cbx3, Chromobox 3), and related proteins. HP1gamma appears to be involved in transcriptional silencing in heterochromatin-like complexes. It binds histone H3 tails methylated at Lys-9, leading to epigenetic repression, and also binds lamin B receptor, an integral membrane protein found in the inner nuclear membrane. The dual binding functions of the protein may explain the association of heterochromatin with the inner nuclear membrane. HP1gamma is also recruited to sites of ultraviolet-induced DNA damage and double-strand breaks. 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. CSD domains have only been found in proteins that also possess a related chromodomain, while chromodomains can exist in isolation. 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. The CSD domains of all three human HP1 homologs have similar affinities to the PXXVXL motif of histone H3.
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