Orc1 Controls Centriole and Centrosome Copy Number in Human Cells

Abstract

Centrosomes, each containing a pair of centrioles, organize microtubules in animal cells, particularly during mitosis. DNA and centrosomes are normally duplicated once prior to cell division to maintain optimal genome integrity. We report a new role for the Orc1 protein, a subunit of the Origin Recognition Complex (ORC) that is a key component of the DNA replication licensing machinery in controlling centriole and centrosome copy number in human cells, independent of its role in DNA replication. Cyclin A promotes Orc1 localization to centrosomes where Orc1 prevents Cyclin Edependent re-duplication of both centrioles and centrosomes in a single cell division cycle. The data suggest that Orc1 is a regulator of centriole and centrosome re-duplication as well as the initiation of DNA replication.

Nonreplicative functions of the origin recognition complex

“…In addition to its participation in several processes within the cell nucleus, ORC has a number of cytoplasmic functions, one of which is an involvement of certain subunits of the complex in the regulation of centrosome duplication [18,19]. …”

“…In mammalian cells, ORC2 depletion leads not only to defects in the DNA replication, but also to abnormalities during cell division, including noticeable defects in chromosome segregation and centrosome division, however, the precise role of ORC in these processes so far remains unclear [43]. The subsequent studies have found that ORC1 contains the centrosomal localization sequence and is directly involved in the regulation of centrosome division, its role being tightly associated with the functions of cyclins [18,19]. The precise mechanism of its action, has not yet been described, although it has been established that ORC1 inhibits the kinase activity of the Cyclin A/CDK2 and Cyclin E/CDK2 complexes through its interaction with cyclins [18,99].…”

The Coordination between DNA Replication Initiation and Other Cell Cycle Events

“…Accordingly, cyclin E can override the ORC1 inhibition of centrosome reduplication, rather than cyclin A or cyclin B. Simultaneous depletion of cyclin E and ORC1 inhibits the reduplication of centrioles caused by ORC1 depletion (Hemerly et al 2009). Moreover, MCM5 also localizes to centrosomes depending on its interaction with CLS domain of cyclin E and prevents centrosome over-duplication in S phase-arrested cells by interacting with cyclin E (Ferguson and Maller 2008).…”

“…The coiled-coil domain of geminin is responsible for its centrosome localization and interaction with Arp1 and is required for the inhibition of centrosome reduplication (Lu et al 2009). Although a number of reports (Hemerly et al 2009, Ferguson and Maller 2008, Tachibana et al 2005, Lu et al 2009) indicate that the same partners in DNA licensing play roles in centrosome duplication, their functional cooperation in centrosome duplication and preventing re-duplication are not yet described.…”

“…These inhibitor proteins, which are either promoted by CDK2/cyclin A to localize at the centrosome, such as ORC1, or directly phosphorylated and recruited to centrosome by CDK2/cyclin E such as MCM5, are enriched at the centrosomes to suppress CDK2/cyclin E activity and prevent centrosome reduplication (Figure 2). Accordingly, depletion of these proteins such as ORC1, ORC2 and geminin leads to centrosome reduplication and multiple centrosome copy numbers www.intechopen.com (Hemerly et al 2009, Prasanth et al 2004, Tachibana et al 2005. Notably, it has been demonstrated that centrosome reduplication by ORC1 depletion is in a cyclin E dependent way, and cyclin E could override the prevention of ORC1 on centrosome over-duplication (Hemerly et al 2009).…”