The intracellular localization of RNAs and proteins often determines cell fates. In developing Drosophila oocytes, the posterior pole localization of oskar (osk) RNA (which requires a polarized microtubule array) and Osk protein directs the assembly and anchoring of the germ plasm at the posterior cortex. The germ plasm is where the factors required for germline and abdomen formation (for example, the RNA helicase Vasa) accumulate. Now, on p. 1107, Tanaka and Nakamura report that the endocytic pathway acts downstream of Osk in Drosophila germ plasm assembly. By screening for mutants in which Vasa accumulation was disrupted, they discover that germ plasm assembly requires the endocytic pathway protein Rabenosyn-5 (Rbsn-5). rbsn-5 mutant oocytes fail to maintain microtubule polarity, which secondarily disrupts osk RNA localization. However, anteriorly misexpressed Osk recruits Rbsn-5 and other endosomal proteins. The researchers suggest that Osk localization at the posterior pole stimulates endosomal cycling, which promotes the F-actin reorganization that anchors the germ plasm components to the oocyte cortex.
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