Nuclear transport

An essential nuclear envelope integral membrane protein, Brr6p, required for nuclear transport. de Bruyn Kops, A. & Guthrie, C. EMBO J. 20, 4183?4193 (2001) [PubMed]

Although the role of the nuclear pore complex in nuclear transport is becoming well established, whether the NE is involved is less clear. de Bruyn Kops and Guthrie report the first example of an essential yeast gene, Brr6, whose product (BRR6) behaves like an integral membrane protein but specifically affects transport of messenger RNA and a protein containing a nuclear export signal. They hypothesize that BRR6 is located adjacent to the nuclear pore and interacts with the pore and transport machinery.

DNA repair

Structure of the Ku heterodimer bound to DNA and its implications for double-strand break repair. Walker, J. R. et al. Nature 412, 607?614 (2001) [PubMed]

The Ku heterodimer (Ku 70 and Ku 80 subunits) maintains genomic integrity by its ability to bind DNA double-strand breaks and facilitate their repair. The authors have solved its crystal structure and find the binding site can cradle two full turns of DNA while encircling only the central 3?4 base pairs. Ku makes no contact with the DNA bases but fits sterically to the contours of the major and minor grooves to position the DNA helix in a defined path through the protein ring. These features seem well designed to support broken DNA ends and aid ligation.

Signal transduction

Gibberellins signal nuclear import of PHOR1, a photoperiod-responsive protein with homology to Drosophila armadillo. Amador, V. et al. Cell 106, 343?354 (2001) [PubMed]

The formation of tubers by some species of potato requires short days and decreased signalling by gibberellins (GA). The authors of this paper have now cloned photoperiod-responsive 1 (PHOR1), which is upregulated under short-day conditions. PHOR1 encodes an armadillo-repeat-containing protein that is rapidly translocated to the nucleus in response to gibberellin application, suggesting that PHOR1 operates in a gibberellin signalling pathway.

Cell adhesion

Integrin-specific activation of Rac controls progression through the G1 phase of the cell cycle. Mettouchi, A. et al. Mol. Cell 8, 115?127 (2001) [Contents page]

In the presence of growth factors, endothelial cells undergo cell-cycle arrest when plated on laminin, but proliferate on fibronectin. This new study shows that on fibronectin, mitogens can mediate Rac activation, resulting in cyclin-D1 accumulation and subsequent G1 to S progression. Adhesion to laminin doesn't mediate Rac activation, indicating that Rac might couple signals generated by specific integrins and mitogens to the cell-cycle machinery.