Gurken (large black dots) and dynein (circled) are found in static sponge bodies in the dorso-anterior portion of the fly oocyte.

RABOUILLE/ELSEVIER

A motor in the fly oocyte is turned into a static anchor to hold its polarity-inducing cargo in place, say Renald Delanoue, Bram Herpers, Jan Soetaert, Ilan Davis (University of Oxford, UK), and Catherine Rabouille (University Medical Centre Utrecht, Netherlands). The switch from motor to anchor is thrown by an RNA-binding protein called Squid.

Squid mutants were originally identified by their polarity defects, which stem from mislocalized gurken mRNA. Gurken is normally transported by Dynein to the dorso-anterior portion of the oocyte. There, it is translated into a signal that instructs the overlying cells to become dorsal. Without Squid, gurken is wrongly dispersed throughout the oocyte anterior.

In the new work, Squid was shown to travel with Dynein and gurken in particles toward the dorso-anterior corner. Upon arrival, particles were transformed into dense, immobile structures called sponge bodies.

The bodies fell apart when Dynein was disrupted, suggesting that the motor becomes a static structural component. Squid was necessary for this transition; its inhibition reverted sponge bodies to transport particles. Squid might help Dynein create such a large complex that it is immobile. Or it might somehow shut off Dynein's motor. Either way, why particles only become sponge bodies at the dorso-anterior corner is a mystery.

Davis says the fly oocyte may seem specialized, “but Dynein is universal. The components we're studying are also likely to be relevant in the nervous system,” where mRNAs are localized and translated far from the cell body.

Reference:

Delanoue, R., et al.
2007
.
Dev. Cell.
13
:
523
–538.