Cell cycle

Inhibition of centrosome protein assembly leads to p53-dependent exit from the cell cycle. Srsen, V. et al. J. Cell Biol. 174, 625–630 (2006)

The authors followed cell-cycle progression after inhibiting centrosome assembly by knocking down the expression of two centrosome-associated proteins, pericentriolar material-1 (PCM1) and pericentrin. Cells failed to enter S phase, yet cells that lacked p53 did not arrest. In addition, inhibiting the p38 mitogen-activated protein kinase rescued cell-cycle progression in the absence of functional centrosomes. Together, this indicates that defective centrosome assembly activates a p53-dependent checkpoint, which requires the p38 stress pathway.

Nuclear transport

Karyopherin-mediated import of integral inner nuclear membrane proteins. King., M.C. et al. Nature 442, 1003–1007 (2006)

How inner nuclear membrane (INM) proteins are targeted to the INM is poorly understood. But, King et al. now provide evidence that the mechanism might be similar to that of soluble proteins. INM proteins have sequences that resemble 'classic' nuclear localization signals. Like nuclear import of soluble proteins, INM-protein import requires Ran GTPase and nuclear transport factors called karyopherins. Specific components of the nuclear pore complex (NPC), termed nucleoporins, also contribute to this process, which implies that the NPC might be adapted to allow the passage of INM proteins.

Cell polarity

CYK-4/GAP provides a localized cue to initiate anteroposterior polarity upon fertilization. Jenkins, N. et al. Science 313, 1298–1301 (2006)

Sequential functioning of the ECT-2 RhoGEF, RHO-1 and CDC-42 establishes cell polarity in Caenorhabditis elegans embryos. Motegi, F. & Sugimoto, A. et al. Nature Cell Biol 8, 978–985 (2006)

Two reports provide insight into how sperm entry initiates cell polarity in the Caenorhabditis elegans one-cell embryo. Jenkins and colleagues showed that the Rho GTPase-activating protein CYK-4 is enriched in sperm, and that paternally donated CYK-4 is essential for polarity along the anterior–posterior axis. The small GTPase RhoA (also known as RHO-1) and the guanine nucleotide-exchange factor ECT-2 are also needed for polarity. They promote myosin light-chain (MLC) activation, which is required for actomyosin contractility. By contrast, CYK-4 inhibits MLC activation and thereby actomyosin contractility. So, sperm entry in the posterior cortex downregulates the actomyosin network locally, and the differential activation of MLC creates a contractile actomyosin gradient. Motegi and Sugimoto showed that ECT-2 somehow gets excluded from the posterior cortex where the sperm enters. This causes the asymmetric distribution of RhoA/RHO-1, which generates an actomyosin gradient to the anterior cortex. This mechanism might work together with the CYK-4 signal to reduce RhoA/RHO-1 activity and establish a contractility gradient that initiates polarity.