Reproduction

A sperm cytoskeletal protein that signals oocyte meiotic maturation and ovulation. Miller, M. A. et al. Science 291, 2144–2147 (2001) [Contents page]

In many animals, oocytes arrest during meiotic prophase. They are triggered to resume meiosis by sperm, which also cause the gonadal sheath cells (needed for ovulation) to contract. Miller and colleagues show here that the major sperm cytoskeletal protein (MSP) is a signal for both of these events. They go on to show that MSP has a function in sperm locomotion, analogous to that of actin, and conclude that MSP has acquired both extracellular signalling and intracellular cytoskeletal functions during evolution.

Protein methylation

Arginine methylation of STAT1 modulates IFNα/β-induced transcription. Mowen, K. A. et al. Cell 104, 731–741 (2001) [contents page]

Previous analysis of STAT transcription factors has focused on their regulation by phosphorylation, shadowing the importance of another modification — arginine methylation. Here, Mowen and colleagues show that STAT1 is methylated on a conserved arginine residue, a modification mediated by the arginine methyltransferase PRMT1 and required for interferon α/β-induced transcription. Moreover, they find that the methyltransferase inhibitor MTA inhibits this methylation, leading to increased association of STAT with its inhibitor PIAS1 and so provide a model for the lack of interferon responsiveness observed in certain cancers.

Cell division

Requirement of a centrosomal activity for cell cycle progression through G1 into S phase Hinchcliffe, E. H. et al. Science 291, 1547–1550 (2001) [contents page]

Centrosome-dependent exit of cytokinesis in animal cells Piel, M. et al. Science 291, 1550–1553 (2001) [contents page]

The centrosome — the main microtubule organizing centre in animal cells — has long been thought to change its microtubule organizing properties in response to cell-cycle progression. These two papers now show that the centrosome, in turn, is required at two distinct stages of the cell cycle — the G1 to S transition and the completion of cytokinesis. Piel and colleagues show that, in animal cells, the centrosome is repositioned after anaphase, and is required for the release of microtubules from the midbody and the completion of cell division. Hinchcliffe et al. removed the centrosome from primate somatic cells, and found that they arrested before S phase, indicating that a centrosomal-associated factor is required for entry into S phase. They also find that, once in mitosis, the centrosome is not required for the G2 to M transition. Together these papers indicate that cells may have checkpoints that monitor centrosome duplication or, say the authors, “core centrosomal structures could bind cell cycle regulatory molecules in a way that activates their function”.