Borealin (green) moves from metaphase chromosomes (left) to the spindle midzone at telophase (right).

Chromosomes are getting crowded. On page 179, Gassmann et al. present their identification of a fourth chromosomal passenger protein. As part of the Aurora B kinase complex, this new protein, Borealin, helps stabilize the mitotic spindle. In a paper soon to appear in Cell, Sampath et al. identify what seems to be the same protein.

Chromosomal passenger proteins, which until now included only Aurora B, INCENP, and Survivin, have several functions during mitosis. As mitosis begins, they are dispersed along chromosomes and phosphorylate histone H3. By metaphase, they gather at centromeres, where they are needed for kinetochore function and to correct spindle attachment errors. Later they move to the spindle midzone and the plasma membrane for cytokinesis.

The identification of Borealin in human cells reveals that not all these jobs are done by the same complex. Borealin was found in a complex containing all three other passengers. Some Aurora B, however, associated with INCENP but not Borealin or Survivin. The smaller complex probably phosphorylates histone H3, as loss of Borealin did not affect this Aurora B function.

As is common for passenger proteins, Borealin localization depended on its partners, INCENP and Survivin. Expression of Borealin fragments, in turn, perturbed the localization of other passengers to the centromeres, but did not affect spindle midzone targeting. Different subcomplexes with or without Borealin may thus bring Aurora B to the appropriate places for its various functions.

Cells that were depleted of Borealin, and that therefore mislocalized the other passengers, were delayed in prometaphase and unable to correct errors in spindle attachments to the kinetochore. Many of these cells nonetheless built normal-looking bipolar spindles but, once anaphase began, formed ectopic asters that caused the chromosomes to segregate in several directions. The extra asters seem to derive from the poles, but the reason for their generation remains unclear. This is the first hint that the chromosomal passengers are involved in spindle assembly and function. ▪