期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2010
卷号：107
期号：34
页码：15093-15098
DOI：10.1073/pnas.1009945107
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：The origin recognition complex (ORC) is a DNA replication initiator protein also known to be involved in diverse cellular functions including gene silencing, sister chromatid cohesion, telomere biology, heterochromatin localization, centromere and centrosome activity, and cytokinesis. We show that, in human cells, multiple ORC subunits associate with hetereochromatin protein 1 (HP1) - and HP1{beta}-containing heterochromatic foci. Fluorescent bleaching studies indicate that multiple subcomplexes of ORC exist at heterochromatin, with Orc1 stably associating with heterochromatin in G1 phase, whereas other ORC subunits have transient interactions throughout the cell-division cycle. Both Orc1 and Orc3 directly bind to HP1, and two domains of Orc3, a coiled-coil domain and a mod-interacting region domain, can independently bind to HP1; however, both are essential for in vivo localization of Orc3 to heterochromatic foci. Direct binding of both Orc1 and Orc3 to HP1 suggests that, after the degradation of Orc1 at the G1/S boundary, Orc3 facilitates assembly of ORC/HP1 proteins to chromatin. Although depletion of Orc2 and Orc3 subunits by siRNA caused loss of HP1 association to heterochromatin, loss of Orc1 and Orc5 caused aberrant HP1 distribution only to pericentric heterochromatin-surrounding nucleoli. Depletion of HP1 from human cells also shows loss of Orc2 binding to heterochromatin, suggesting that ORC and HP1 proteins are mutually required for each other to bind to heterochromatin. Similar to HP1-depleted cells, Orc2 and Orc3 siRNA-treated cells also show loss of compaction at satellite repeats, suggesting that ORC together with HP1 proteins may be involved in organizing higher-order chromatin structure and centromere function.
