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Title: Stabilization of p27Kip1/CDKN1B by UBCH7/UBE2L3 catalyzed ubiquitinylation: a new paradigm in cell-cycle control

Author
item WHITCOMB, ELIZABETH - Jean Mayer Human Nutrition Research Center On Aging At Tufts University
item TSAI, YIEN CHE - National Cancer Institute (NCI, NIH)
item BASAPPA, JOHNVESLY - University Of Pennsylvania
item LIU, KE - Sichuan University
item LEFEUVRE, AURELIE - Mission Therapeutics
item WEISSMAN, ALLAN - National Cancer Institute (NCI, NIH)
item TAYLOR, ALLEN - Jean Mayer Human Nutrition Research Center On Aging At Tufts University

Submitted to: Journal of Federation of American Societies for Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2018
Publication Date: 8/16/2018
Citation: Whitcomb, E., Tsai, Y., Basappa, J., Liu, K., Le Feuvre, A., Weissman, A.M., Taylor, A. 2018. Stabilization of p27Kip1/CDKN1B by UBCH7/UBE2L3 catalyzed ubiquitinylation: a new paradigm in cell-cycle control. Journal of Federation of American Societies for Experimental Biology. https://doi.org/10.1096/fj.201800960R.
DOI: https://doi.org/10.1096/fj.201800960R

Interpretive Summary: We introduce a new counterintuitive aspect of ubiquitination: stabilization of substrates. Ubiquitin can be attached to target proteins either as a monomer or as a chain of ubiquitins and usually directs substrates for proteolytic degradation. The cell cycle is controlled by precisely timed proteasomal degradation of regulatory molecules. We demonstrate here that rather than catalyzing degradation, an enzyme in the ubiquitin pathway, UBCH7, protects a crucial cell cycle regulatory protein, p27Kip1, from degradation. Over-expression of UBCH7 increases the level of the p27Kip1 and delays entry into the cell cycle. Depletion of UBCH7 leads to diminished p27Kip1 levels. We show that UBCH7 catalyzes the attachment of ubiquitin chains which do not target p27Kip1 for degradation, leading to its stabilization.

Technical Abstract: Ubiquitination drives many cellular processes by targeting proteins for proteasomal degradation. Ubiquitin conjugation enzymes (Ubc) promote ubiquitination and degradation of protein substrates, thus, higher levels of Ubcs are usually related to decreased levels of their substrates. Progression through the cell cycle is tightly controlled by the activities of cyclins, cyclin dependent kinases (CDK) and CDK inhibitors (CDKI). Concentrations of the major cell cycle regulatory proteins, including the CDKI p27Kip1, are primarily regulated by ubiquitination and degradation via the ubiquitin proteasome system (UPS). We recently reported that UBCH7/UBE2L3 controls the entry into and progression through S-phase of the cell cycle. Here we show that in contrast with promoting degradation, UBCH7/UBE2L3 specifically protects p27Kip1 from UPS-dependent degradation. Thus, over expression of UBCH7/ UBE2L3 stabilizes p27Kip1 and delays the G1 to S transition, while depletion of UBCH7/UBE2L3 decreases steady-state levels of p27Kip1. Levels of p21Cip1/Waf1, p57Kip2, cyclin A and cyclin E, all of which are also involved in regulating the G1/S transition are not affected by UBCH7/UBE2L3 depletion. The effect of UBCH7/UBE2L3 on p27Kip1 is not due to alteration of the levels of any of the E3s known to ubiquitinate p27Kip1. Rather, UBCH7/UBE2L3 catalyzes the conjugation of diverse ubiquitin chains on p27Kip1 which appear to be proteolytically incompetent. These data reveal new controls and concepts about the UPS in which a ubiquitin conjugating enzyme selectively inhibits and may even protect, rather than promote degradation of a crucial cell cycle regulatory molecule.