The polarity of the Drosophila oocyte and embryo is set up through the asymmetric localisation of several mRNAs in the female egg. Whereas the mRNAs located in the dorsal-anterior region are localised by microtubule-minus-end-directed dynein motor proteins, the posteriorly localised oskar mRNA primarily relies on plus-end-directed kinesin for its proper positioning. Kinesin-1 consists of the heavy chain motor subunit (Khc) and the Kinesin light chain adaptor (Klc), which is thought to be dispensable for the transport of several cargoes. In this issue (p. 4252), Graydon Gonsalvez and colleagues probe for interacting partners for Khc with a mass spectrometry approach; one identified proteins is a new isoform of non-muscle tropomyosin 1 (Tm1C), which interacts with the C-terminal domain of Khc. The depletion of Tm1C results in oskar mRNA losing its specific accumulation at the posterior pole of the oocyte. Importantly, the authors show that a number of defects known to occur upon loss of Khc in oocytes are not observed after Tm1C depletion: the oocyte nucleus positions correctly, no aberrant actin spheres are formed and gurken and bicoid mRNA are correctly localised in the absence of Tm1C. The authors also show that, unlike oskar mRNA, TmC1 is not restricted to the female germline, which raises the possibility that other cell types might use Khc and Tm1C for cargo transport. Taken together, this work demonstrates that Kinesin-1 works together with a previously uncharacterised tropomyosin isoform to establish cell polarity in Drosophila oocytes.

• © 2016. Published by The Company of Biologists Ltd