It seems that the mitotic spindle is important for segregating more than just a cell's chromosomes. Wei and Seemann reveal how a dividing cell uses the spindle to ensure that both of its daughter cells inherit a fully functional Golgi apparatus.
In mammalian cells, individual Golgi stacks assemble together into a single continuous ribbon. This is important for cell polarity and migration because the entire ribbon can be oriented to direct membrane traffic to a specific site within the cell. At the beginning of mitosis, the Golgi fragments into small tubules and vesicles that cluster around the spindle poles. The fragments then separate into the daughter cells before reforming the ribbon, but it wasn't known whether the spindle machinery was actually required for their partition.
Wei and Seemann took a difficult but direct approach to investigating this question: they made cells without mitotic spindles. Using a nifty series of manipulations, the authors were able to make cells divide so that one daughter received none of the spindle and therefore ended up with no DNA, no centrosomes, and no microtubules. Surprisingly, these unfortunate cells did contain small, scattered Golgi stacks capable of secreting protein, but the stacks weren't organized into a larger ribbon structure. The ribbon could be reassembled in these cells, however, if they were provided with a protein extract prepared from purified Golgi and some tubulin to make microtubules. Alternatively, allowing cells to inherit just a little bit of the mitotic spindle also enabled them to form a Golgi ribbon.
The researchers think that while Golgi stacks are inherited independently of the spindle, certain Golgi proteins essential for building the ribbon are segregated using the mitotic machinery, ensuring that both daughters can form a Golgi ribbon and polarize their membrane transport. The next challenge is to identify what these “ribbon determinants” are. The team already knows that the Golgi extract used in their experiments was enriched in trans-Golgi proteins, so one approach, says author Joachim Seemann, will be to add individual candidates back into their system to see whether ribbon formation can be rescued.
