It takes two crews of proteins to keep the telomeres in fine 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 telomere 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 finding points to drugs that break up the complex as a possible treatment for cancer.

Kim, S.-h., et al.
J. Cell Biol.