SCAPER (red) keeps cyclin A (green) in the cytoplasm to promote S phase progression.

Cyclin A promotes the progression of both S phase and mitosis. A new cyclin A binding protein, identified by Tsang et al. (page 621), helps the S phase push but hinders mitotic progression. The new partner seems to favor S phase by holding cyclin A in the cytoplasm.

To determine how cyclin A pushes forward different stages of the cell cycle, Dynlacht's team looked for new binding partners. They fished out one especially strong candidate, which was associated with the ER, and named it SCAPER (S phase cyclin A–associated protein of the endoplasmic reticulum).

Overexpression of SCAPER delayed progression of cells through M phase, suggesting that the protein blocks this cyclin A activity. But SCAPER deletion prevented cells from entering S phase in a timely manner, conversely indicating that it...

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