Tag team at the telomeres

Tag team at the telomeres I t takes two crews of proteins to keep the telomeres in fi ne fettle, as Kim et al. show. TRF1 and TRF2 are just two of the proteins that ensure that a cell's telomeres remain long and structurally sound. Another protein, TIN2, links up with both molecules and with other proteins to form complexes that help maintain the telomeres. However, researchers weren't sure whether TRF1 and TRF2 worked together in the same complex. When Kim et al. netted TIN2-containing complexes from nuclear extracts of human cells, they found that TRF1 and TRF2 usually separated. TRF1 turned up in what the scientists dubbed complex A, and TRF2 appeared in complex B. Further evidence that TRF1 and TRF2 go their own way came when the researchers altered cells that produce normal TIN2 to also make either of two TIN2 mutants. One of the mutants, which can't attach to TRF1 but can latch onto TRF2, bound to and broke up the B complexes. The second mutant, TIN2, which can't hook onto TRF2 but can bind to TRF1, disrupted A complexes. The work indicates that the A and B complexes perform different jobs. The tail of a telomere doubles over on itself, a process called capping. Complex B appears to control capping, whereas complex A might help the telo-mere keep in shape. However, the researchers say they still haven't ruled out the possibility that the A and B complexes combine. The team also discovered that disrupting the B complex was lethal for cells missing the antitumor protein p53, which is absent from many cancer cells. That fi nding points to drugs that break up the complex as a possible treatment for cancer. W hether a cell is on the move or tangling with invading bacteria, the enzyme Rac1 is on the job. Michaelson et al. add another task to the hard-working molecule's resume. It shuttles into and out of the nucleus to help induce mitosis. By reshaping the actin cytoskeleton, Rac1 prompts cells to crawl, polarize, or form adherens junctions with their neighbors. It also switches on genes and controls the activity of an antipathogen enzyme. Although researchers had observed Rac1 in the nucleus when it was overexpressed, they thought that the enzyme exerted most of its effects from the cytoplasm. But Michaelson et al. found that Rac1 enters and departs from the nucleus at particular points in …