Rac1 (white) is in the cytoplasm for G1 (left) but heads to the nucleus for G2 (right).

Whether 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 hardworking 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 the cell cycle. The protein builds up in the nucleus during the G2 phase. By the next G1 phase, it's back in the cytoplasm. Rac1's entry into the nucleus promoted cell division, the team found. Conversely, a Rac1 variant that can't access the nucleus hindered mitosis.

What directs Rac1 into the nucleus at G2 isn't clear. The team's findings suggest that to hold Rac1 in the cytoplasm, cells attach a prenyl lipid group to the protein's tail. However, the researchers also showed that much of the Rac1 that reaches the nucleus carries the prenyl group. The researchers speculate that a protein chaperone might cover up the lipid and allow Rac1 to make its move.

Why Rac1 enters the nucleus is also mysterious. It might arrive to activate some of the molecular machinery of mitosis. Alternatively, cells could be exiling the protein from the cytoplasm because many of its functions, such as promoting junction formation and cell spreading, hamper division.

Michaelson, D., et al.
J. Cell Biol.