Molecular basis of mRNA transport by a kinesin-1–atypical tropomyosin complex

Kinesin-1 carries cargos including proteins, RNAs, vesicles, and pathogens over long distances within cells. The mechanochemical cycle of kinesins is well described, but how they establish cargo specificity is not fully understood. Transport ofoskarmRNA to the posterior pole of theDrosophilaoocyte is mediated byDrosophilakinesin-1, also called kinesin heavy chain (Khc), and a putative cargo adaptor, the atypical tropomyosin,aTm1. How the proteins cooperate in mRNA transport is unknown. Here, we present the high-resolution crystal structure of a Khc–aTm1 complex. The proteins form a tripartite coiled coil comprising two in-register Khc chains and oneaTm1 chain, in antiparallel orientation. We show thataTm1 binds to an evolutionarily conserved cargo binding site on Khc, and mutational analysis confirms the importance of this interaction for mRNA transport in vivo. Furthermore, we demonstrate that Khc binds RNA directly and that it does so via its alternative cargo binding domain, which forms a positively charged joint surface withaTm1, as well as through its adjacent auxiliary microtubule binding domain. Finally, we show thataTm1 plays a stabilizing role in the interaction of Khc with RNA, which distinguishesaTm1 from classical motor adaptors.