Orc2 protects ORCA from ubiquitin-mediated degradation

• Published in: Cell Cycle
• Main Authors: Shen, Zhen, Prasanth, Supriya G.
• Format: Text
• Language: English
• Published: Landes Bioscience 2012
• Subjects: Report; Orca
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3478309; http://www.ncbi.nlm.nih.gov/pubmed/22935713; https://doi.org/10.4161/cc.21870

Origin recognition complex (ORC) is highly dynamic, with several ORC subunits getting posttranslationally modified by phosphorylation or ubiquitination in a cell cycle-dependent manner. We have previously demonstrated that a WD repeat containing protein ORC-associated (ORCA/LRWD1) stabilizes the ORC on chromatin and facilitates pre-RC assembly. Further, ORCA levels are cell cycle-regulated, with highest levels during G1, and progressively decreasing during S phase, but the mechanism remains to be elucidated. We now demonstrate that ORCA is polyubiquitinated in vivo, with elevated ubiquitination observed at the G1/S boundary. ORCA utilizes lysine-48 (K48) ubiquitin linkage, suggesting that ORCA ubiquitination mediates its regulated degradation. Ubiquitinated ORCA is re-localized in the form of nuclear aggregates and is predominantly associated with chromatin. We demonstrate that ORCA associates with the E3 ubiquitin ligase Cul4A-Ddb1. ORCA is ubiquitinated at the WD40 repeat domain, a region that is also recognized by Orc2. Furthermore, Orc2 associates only with the non-ubiquitinated form of ORCA, and Orc2 depletion results in the proteasome-mediated destabilization of ORCA. Based on the results, we suggest that Orc2 protects ORCA from ubiquitin-mediated degradation in vivo.