In Brief

Signal transduction

Dishevelled activates Ca²⁺ flux, PKC, and CamKII in vertebrate embryos. Sheldahl, L. C. et al. J. Cell Biol. 161, 769–777 (2003)

Wnt signalling through Frizzled (Fz) receptors can activate the planar cell polarity (PCP), β-catenin and Ca²⁺ pathways. Dishevelled (Dsh) functions in the β-catenin and PCP pathways, but this study now shows that Dsh also functions in the Ca²⁺ pathway — it activates Ca²⁺ flux, protein kinase C (PKC) and Ca²⁺/calmodulin-dependent kinase II. Indeed, Dsh function is required for full activation of PKC by Fz7 in Xenopus eggs.

Centrosomes

Centrosome number is controlled by a centrosome-intrinsic block to reduplication. Wong, C. & Stearns, T. Nature Cell Biol. 5, 539–544 (2003)

Centrosomes duplicate only once during the cell cycle but it has been unknown whether this is because of a decline in positive factors or the result of a specific reduplication block. Here, Wong and Stearns used cell-fusion assays to show that a centrosome-intrinsic block, rather than any cytoplasmic factors, prevents reduplication of recently duplicated G2 centrosomes, and that this block is not controlled by the centrosome:nucleus ratio.

Telomeres

Developmentally programmed gene elimination in Euplotes crassus facilitates a switch in the telomerase catalytic subunit. Karamysheva, Z. et al. Cell 113, 565–576 (2003)

Here, Karamysheva et al. show that three TERT genes encode the catalytic subunit of telomerase in Euplotes crassus. Their expression requires +1 ribosomal frameshifting. TERT-1 and TERT-3 expression correlates with telomere maintenance, whereas TERT-2 is expressed during de novo telomere formation. TERT-2 expression is controlled by programmed DNA degradation, so that it is eliminated during vegetative growth.

Signalling

A hedgehog-responsive region in the Drosophila wing disc is defined by Debra-mediated ubiquitination and lysosomal degradation of Ci. Dai, P. et al. Dev. Cell 4, 917–928 (2003)

The Hh-responsive genes Decapentaplegic and Patched are expressed specifically in a stripe of A-compartment cells 9–10 cells away from the A–posterior border. The transcription factor Cubitus interruptus (Ci), which activates Hh target genes, is present at high levels in this stripe, too. Dai et al. have now identified Debra (Dbr) — for determiner of breaking down of Ci activator — which binds to Ci and mediates Ci polyubiquitylation. This results in Ci lysosomal degradation at the border of the Hh-responsive region.