Cell migration

Two distinct actin networks drive the protrusion of migrating cells. Ponti, A. et al. Science 305, 1782–1786 (2004)

Cell migration requires plasma-membrane protrusion, which is brought about by actin polymerization at the leading edge of the cell. Using quantitative fluorescent speckle microscopy, Ponti et al. were able to distinguish between two actin networks that colocalized at the leading edge of epithelial cells. A lamellipodium network promoted random protrusion and retraction, whereas the lamella caused productive cell advance, thanks to the coupling of actomyosin contractile forces to substrate adhesion.

Developmental cell biology

Mesenchymal–epithelial transition during somitic segmentation is regulated by differential roles of Cdc42 and Rac1. Nakaya, Y. et al. Dev. Cell 7, 425–438 (2004)

The authors studied the effects of different levels of Rac1 and Cdc42 activity on the transition between mesenchymal and epithelial states of cells during somitogenesis in chick embryos. The level of Cdc42 activity was important for the decision between epithelial and mesenchymal cell states. Specific levels of Rac1 activity were necessary for correct epithelialization.

Cell growth

A dynamic transcriptional network communicates growth potential to ribosome synthesis and critical cell size. Jorgensen, P. et al. Genes Dev. 18, 2491–2505 (2004)

Cells need to reach a critical cell size before they commit to cell division (a point called Start in budding yeast). The authors identified two Start repressors — Sfp1 and Sch9 — that are also activators of a transcriptional network that regulates ribosome biogenesis. The abundance of Sch9 and the nuclear localization of Sfp1 were regulated by nutrient status. In turn, Sfp1 regulates ribosome synthesis in response to nutrients. So, nutrient status determines the critical cell-size threshold by Sfp1- and Sch9-mediated control of ribosome synthesis.

Chromatin

Human SirT1 interacts with histone H1 and promotes formation of facultative heterochromatin. Vaquero, A. et al. Mol. Cell 16, 93–105 (2004)

Vaquero and colleagues identified SirT1 as a human histone deacetylase with a preference for histone H4 lysine 16 (H4-K16), and showed that SirT1 interacts with, and deacetylates, H1-K26. SirT1-mediated gene silencing seems to involve a functional role for H1 — expression of a SirT1 reporter resulted in the deacetylation of H4-K16, the recruitment of H1 to the promoter and the subsequent loss of H3-K79 methylation, which is characteristic of heterochromatin.