Plant HP1 Protein ADCP1 Links Multivalent H3K9 Methylation Readout To Heterochromatin Formation

Eukaryotic genomes are organized and compressed into chromatin,which is mainly divided to two types: euchromatin and heterochromatin.Heterochromatin is highly condensed,and the compact structure is essential for maintaining chromosome intensity and stability,and transposons silencing.Heterochromatin dysfunction can induce genetic turmoil and is closely associated with aging and tumorigenesis.HP1(Heterochromatin protein 1)binds to the conserved heterochromatic marker H3K9 methylation,shapes the structure basis for the condensed heterochromatin,and is actively turnover between chromatin-bound and free states,which is required for the function of heterochromatin.But HP1 proteins have never been reported in plants yet.HP1 is evolutionally conserved from yeast to mammals.However,HP1 function is not conserved in plants.In Arabidopsis,the homolog of HP1,called LHP1(Like Heterochromatin Protein 1)is the reader of H3K27me3.Therefore,identification the heterochromatin reader and its molecular mechanisms is necessary in plants.In this study,we identify a novel H3K9me2 reader ADCP1,which is the HP1 functional protein in plant and links multivalent H3K9me2 readout to heterochromatin formation.After the H3K9me2 peptide binding proteins which belong to Agenet domain containing proteins(called ADCP1 and ADCP2)were isolated by our cooperators,and they found ADCP1 could drive phase separation of H3K9me3 modified nucleosome array,we processed in-depth study of ADCPs' reader functions in vivo.First,we performed immunostaining and Ch IP-seq to detect the genomic binding of ADCP1,and confirmed the co-localization of H3K9me2 and ADCP1.Second,through ADCP1 knockout and overexpression,we found ADCP1 was important for the heterochromatic chromocenter formation.Subsequently,by high-throughput sequencing,we found ADCP1 was required for the maintenance of H3K9me2 and CHG/CHH DNA methylation,as well as transposon silencing.Finally,we identified the interaction between ADCP1 and a high mobility protein HMGA,and ADCP1 could be regulated by chromatin structure factors,which provided clues for the exploring of the mechanisms.Besides,we also investigated the functions of ADCP2 and speculated that ADCP2 may be an organism-specific heterochromatic regulator.In conclusion,we characterized a novel plant H3K9me2 reader,and revealed its important roles in heterochromatin formation,which reflected the complicated and conserved epigenetic regulation mechanisms among different species.This work can provide new ideas to study other crops,such as maize and rice which contain higher proportion of transposons.