Nuclear transport

The COPI complex functions in nuclear envelope breakdown and is recruited by the nucleoporin Nup153. Liu, J. et al. Dev. Cell 5, 487–498 (2003)

Even though vesiculation has been largely discounted in having a role in nuclear-envelope breakdown, Liu and colleagues now show that vesicle formation by a COPI-mediated pathway could, in fact, be an important step in this process. They found that the nucleoporin Nup153, which turned out to be crucial for nuclear-envelope breakdown, associates with members of the COPI coatamer complex and is important in directing COPI to the nuclear membrane during mitosis, when nuclear disassembly occurs.

Signalling

hippo encodes a Ste-20 family protein kinase that restricts cell proliferation and promotes apoptosis in conjunction with salvador and warts. Wu, S. et al. Cell 114, 445–456 (2003)

The Drosophila Mst ortholog, hippo , restricts growth and cell proliferation and promotes apoptosis. Harvey, K. F., Pfleger, C. M. & Hariharan, I. K. Cell 114, 457–467 (2003)

The authors of both of these papers report the characterization of Hippo, a serine/threonine protein kinase of the Ste20 kinase family. Hippo seems to regulate organ size through its ability to regulate cell proliferation and apoptosis. Hippo works in allegiance with the Warts protein kinase and the Salvador tumour suppressor. Phosphorylation of Salvador by Hippo promotes Hippo phosphorylation of Warts. The ternary complex promotes apoptosis by downregulating levels of the caspase antagonist DIAP1, although whether this occurs through a transcriptional or post-translational mechanism is unclear. Loss of Hippo function results in increased levels of the cell-cycle regulator cyclin E, indicating that Hippo restricts cell proliferation by negatively regulating cyclin E.

Cytoskeleton

How capping protein binds the barbed end of the actin filament. Wear, M. A. et al. Curr. Biol. 13, 1531–1537 (2003)

Capping protein (CP) can cap the barbed end of actin filaments, and a 'tentacle' model has previously proposed that the carboxyl termini of the α and β subunits of CP are involved in this capping. Now, Wear et al. show that CP does seem to use two independent actin-binding tentacles to cap actin, and that the α tentacle is more important than the β tentacle. In addition, as CP with either tentacle can cap actin, they propose that, rather than contacting one actin monomer at the end of a protofilament, each tentacle contacts two or three monomers and lies at the interface between the two protofilaments.