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Molecular and Cellular Biology, May 2001, p. 3256-3265, Vol. 21, No. 9
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.9.3256-3265.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Accumulation of Cyclin E Is Not a Prerequisite for Passage through the Restriction Point

Susanna V. Ekholm,1,2 Peter Zickert,1 Steven I. Reed,2 and Anders Zetterberg1,*

Department of Oncology-Pathology, Cellular and Molecular Tumorpathology, Karolinska Institute/Karolinska Hospital, S-171 76 Stockholm, Sweden,1 and Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 920372

Received 18 October 2000/Returned for modification 27 November 2000/Accepted 9 February 2001

The restriction point (R) is defined as the point in G1 after which cells can complete a division cycle without growth factors and divides G1 into two physiologically different intervals in cycling cells, G1-pm (a postmitotic interval with a constant length of 3 to 4 h) and G1-ps (a pre-DNA-synthetic interval with a variable length of 1 to 10 h). Cyclin E is a G1 regulatory protein whose accumulation has been suggested to be critical for passage through R. We have studied cyclin E protein levels in individual cells of asynchronously growing cell populations, with respect to both passage through R and entry into S phase. We found that the postmitotic G1 cells that had not yet reached R were negative for cyclin E accumulation. On the other hand, cells that had passed R were found to accumulate cyclin E at variable times (1 to 8 h) after passage through R and 2 to 5 h before entry into S. These kinetic data rule out the hypothesis that passage through R is dependent on the accumulation of cyclin E but suggest, instead, the converse, that passage through R is a prerequisite for cyclin E accumulation. Furthermore, we found that most of the cyclin E protein is downregulated within 1 to 2 h after entry into S.

* Corresponding author. Mailing address: Department of Oncology and Pathology, Karolinska Institute, Karolinska Sjukhuset, CCK R8:04, 171 76 Stockholm, Sweden. Phone: (46) 851-77-5250. Fax: (46) 832-1047. E-mail: anders.zetterberg{at}onkpat.ki.se.

Molecular and Cellular Biology, May 2001, p. 3256-3265, Vol. 21, No. 9
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.9.3256-3265.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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