Partitioning endosomes

Endosomes carrying an activated receptor are divided equally by joining the mitotic spindle, as shown by Christian Bökel (Max Planck Institute, Dresden, Germany), Marcos González-Gaitán (Geneva University, Switzerland), and colleagues. Daughter cells thus start out on an equal signaling footing.

An equal share is key for cells that are reading ligand concentration gradients. The Dpp gradient, for example, helps developing fly wing cells to detect their position. Dpp and its bound receptor, called Tkv, are now shown to reside in a set of early endosomes that line up at the spindle midzone during anaphase.

By cytokinesis, the endosomes were found in two groups on either side of the midzone. The group is now trying to identify the molecular links between endosomes and the spindle. They hope to identify the mechanism that ensures that centrally located endosomes are partitioned equally.

Dpp and Tkv get to these partitioning endosomes via the Sara adaptor protein. Sara mutations left Tkv off the spindle and daughter cells with variable levels of Dpp signaling activity. Over time, however, the differences were dampened. González suggests that the weaker daughter is strengthened by rereading Dpp levels. Apoptosis pruned cells with too much signal.

Wing cells divide symmetrically, so the authors now want to test whether asymmetrically dividing cells use the spindle to divvy up unequal portions of receptors.

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