Novel control of S-phase of the cell cycle by ubiquitin conjugating enzyme H7

Authors: WHITCOMB, ELIZABETH - JM USDA HNRCA @ TUFTS; DUDEK, EDWARD - JM USDA HNRCA @ TUFTS; LIU, QING - BRIGHAM & WOMEN'S HOSP; Taylor, Allen

Submitted to: Molecular Biology of the Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2008
Publication Date: 1/1/2009
Citation: Whitcomb, E., Dudek, E.J., Liu, Q., Taylor, A. 2009. Novel control of S-phase of the cell cycle by ubiquitin conjugating enzyme H7. Molecular Biology of the Cell. 20(1):1-9.

Interpretive Summary: The ubiquitin proteolytic pathway is important for regulating cell division. There are many enzymes which are involved in putting ubiquitin on other proteins and, in so doing, to regulate cell cycle. To date, there is no information about most of these. We have investigated the role of one ubiquitin conjugating enzyme, UbcH7, in controlling cell division. We show that in several different types of cells, including lens and HeLa cells, UbcH7 protein levels are decreased during the time when DNA is replicated, or S phase. The level of UbcH7 protein recovers before the cells divide in mitosis. The changes in UbcH7 protein levels appear to be controlled by a ubiquitin dependent proteolytic process and control of this proteolytic process can be used to control the cell cycle. Together the data indicate for the first time that UbcH7 controls cell cycle UbcH7 and the level of this enzyme is itself controlled by the ubiquitin pathway. Depletion affects the levels of several proteins which are involved in controlling DNA damage and cell proliferation. These changes in the levels of other proteins could be the cause of the alteration in the cell cycle profile observed upon UbcH7 manipulation.

Technical Abstract: Timely degradation of regulatory proteins by the ubiquitin proteolytic pathway (UPP) is an established paradigm of cell cycle regulation during the G2/M and G1/S transitions. Less is known about roles for the UPP during S phase. Here we present evidence that dynamic cell cycle dependent changes in levels of UbcH7, regulate entrance into and progression through S phase. In diverse cell lines, UbcH7 protein levels are dramatically reduced in S phase but are fully restored by G2. Knockdown of UbcH7 increases the proportion of cells in S phase and doubles the time to traverse S phase whereas UbcH7 over expression reduces the proportion of cells in S phase. These data suggest a role for UbcH7 targets in the completion of S phase and entry into G2. Notably, UbcH7 knockdown was coincident with elevated levels of the checkpoint kinase Chk1 but not Chk2. These results argue that UbcH7 promotes S phase progression to G2 by modulating the intra-S phase checkpoint mediated by Chk1. Furthermore, UbcH7 levels appear to be regulated by a UPP. Together the data identify novel roles for the UPP, specifically UbcH7 in the regulation of S phase transit time as well as in cell proliferation.