A cell lacking centrosomes (left) can still undergo mitosis (right).

Centrosomes are dynamic leaders that propel the cell cycle forward. Or are they followers swept along by change? On page 173, Uetake et al. offer evidence that reconciles these disparate views. The group demonstrates that, although centrosomes are not required for cell cycle progression, their loss halts the process under stressful conditions.

Researchers long thought that centrosomes were passive participants in the cell cycle. But several recent studies found that removing or injuring centrosomes prevents cells from entering S phase, suggesting that the centrosome isn't just along for the ride.

To resolve this uncertainty, Uetake et al. excised the centrosomes from two kinds of normal cells. The cells nicely rolled through G1 into S phase, suggesting that centrosomes are not normally vital for entering S phase.

To explain why previous reports found differently, the authors considered stressed cells, which often stall in G1. They proposed that centrosome loss might put cells under small amounts of pressure that create stalls when added to other stresses.

Uetake et al. tested the idea by removing centrosomes from cells and then exposing them to blue light stress. Neither stimulus alone halted the cell cycle, but the combination did. Cells exposed to both stresses could advance through G1 if the researchers first blocked p38, which switches on a G1 arrest when cells are under duress.