Controlling traffic at cell junctions

Polarised exocytosis is essential for both the establishment and maintenance of cell polarity. This is particularly apparent during the junctional remodelling that accompanies cell rearrangements during axis elongation. The tethering complex exocyst plays a key role in trafficking vesicles to the plasma membrane but the receptors that lend specificity, and thus polarity, to this process are not well understood. Now, Nic Tapon and colleagues show that the adherens junction component RASSF8 acts as a receptor for the exocyst during proximo-distal (PD) extension of the Drosophila pupal wing. They find RASSF8 mutants fail to extend their PD axis due to defects in cell rearrangements. They also show that the exocyst component Sec15 is a binding partner of RASSF8, and that Sec15- and Rab11-positive vesicles accumulate in the cytoplasm in RASSF8 mutants. The authors further identify that the adhesion protein Echinoid is a cargo protein in these stranded vesicles and confirm that loss of Echinoid via RNAi causes defective PD axis extension. Together, these data identify a new exocyst-receptor pairing that lends specificity to exocyst-dependent plasma membrane trafficking.