Cell cycle

Calcium, calmodulin, and CaMKII requirement for initiation of centrosome duplication in Xenopus egg extracts. Matsumoto, Y. & Maller, J. L. Science 295, 499?502 (2002) [PubMed]

Centrosomes are duplicated once ? and only once ? per round of cell division. Studies from Xenopus egg extracts have shown that the cyclin-dependent kinase 2 (Cdk2) is required for many rounds of centrosome duplication. However, inactivation of Cdk2 does not block the initial duplication event. Matsumoto and Maller now show that chelation of calcium, or specific inactivation of the calcium/calmodulin-dependent protein kinase II (CaMKII) blocks all centrosome duplications, including the first one.

Nuclear architecture

Directed proteomic analysis of the human nucleolus. Andersen, J. S. et al. Curr. Biol. 12, 1?11 (2002) [PubMed]

Paraspeckles: a novel nuclear domain. Fox, A. H. et al. Curr. Biol. 12, 13?25 (2002) [PubMed]

The most important function of the nucleolus is in the generation of ribosomal subunits, but could it have other functions? To address this, the authors of these papers undertook a proteomic analysis of human nucleoli. They identified over 250 proteins, two-thirds of which (including ribosomal proteins that process factors and other transcriptional components) have functions that are associated with the known role of the nucleolus. Among the new proteins, Paraspeckle protein 1 (PSP1), PSP2 and p54/nrb were found to accumulate in new compartments ? paraspeckles ? that correspond to discrete regions in the heterochromatin nucleoplasmic space. All primary and transformed cell lines that were studied contained 10?20 paraspeckles. The three proteins move dynamically between paraspeckles and the nucleolus, but relocalise in nucleolar caps within the nucleolus when transcription is blocked. Combined with the fact that they all have related RNA-binding domains, this indicates that they might have a role in transcriptional regulation.

Signal transduction

Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase. Minokoshi, Y. et al. Nature 415, 339?343 (2002) [PubMed]

Leptin is secreted by adipocytes to lower appetite and increase energy expenditure, but the signals that mediate these effects are not well known. Here, the authors report that leptin can prevent lipids accumulating in non-adipose tissue by stimulating the phosphorylation and activation of 5'-AMP-activated protein kinase (AMPK). By inhibiting acetyl coenzyme A carboxylase activity, AMPK stimulates fatty-acid oxidation, initially by a direct effect on the cell, but later requires the hypothalamic?sympathetic nervous system axis.