The Kinesin-1 binding domain of Ensconsin/MAP7 promotes Kinesin-1 activation in vivo

Centrosome separation in Drosophila larval neuroblasts and asymmetric transport of embryonic determinants in oocytes are both microtubule-dependent processes that require Kinesin-1 activation by Ensconsin/microtubule-associated protein 7 (MAP7). However, the molecular mechanism used by Ensconsin to activate Kinesin-1 remains elusive. Ensconsin/ MAP7 contains an N-terminal microtubule-binding domain (MBD) and a C-terminal Kinesin-binding domain (KBD). Using rescue experiments in live flies, we show that KBD expression alone is sufficient to restore the correct localization patterns of Staufen and Gurken proteins in oocytes. Moreover, in neural stem cells, this domain is also sufficient to fully rescue Ensconsin-dependent centrosome separation defects. Therefore, we propose that the KBD/Kinesin-1 motor represents a minimal activation module that is sufficient to promote Kinesin-1 activation in most Drosophila cells. Nonetheless, our data also suggest that in a very large cell with a complex microtubule network, this minimal module is not sufficient for optimal Kinesin-1 function that requires direct targeting of Kinesin-1 to microtubules by Ensconsin.