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

Abstract

ABSTRACT The restriction point (R) is defined as the point in G 1 after which cells can complete a division cycle without growth factors and divides G 1 into two physiologically different intervals in cycling cells, G 1 -pm (a postmitotic interval with a constant length of 3 to 4 h) and G 1 -ps (a pre-DNA-synthetic interval with a variable length of 1 to 10 h). Cyclin E is a G 1 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 G 1 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.