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

AUTOR(ES)

Ekholm, Susanna V.

FONTE

American Society for Microbiology

RESUMO

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.

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