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 of oskar mRNA to the posterior pole of the Drosophila oocyte is mediated by Drosophila kinesin-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 one aTm1 chain, in antiparallel orientation. We show that aTm1 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 with aTm1, as well as through its adjacent auxiliary microtubule binding domain. Finally, we show that aTm1 plays a stabilizing role in the interaction of Khc with RNA, which distinguishes aTm1 from classical motor adaptors.

A translation-independent role of oskar RNA in early Drosophila oogenesis.

The Drosophila maternal effect gene oskar encodes the posterior determinant responsible for the formation of the posterior pole plasm in the egg, and thus of the abdomen and germline of the future fly. Previously identified oskar mutants give rise to offspring that lack both abdominal segments and a germline, thus defining the ;posterior group phenotype'. Common to these classical oskar alleles is that they all produce significant amounts of oskar mRNA. By contrast, two new oskar mutants in which oskar RNA levels are strongly reduced or undetectable are sterile, because of an early arrest of oogenesis. This egg-less phenotype is complemented by oskar nonsense mutant alleles, as well as by oskar transgenes, the protein-coding capacities of which have been annulled. Moreover, we show that expression of the oskar 3' untranslated region (3'UTR) is sufficient to rescue the egg-less defect of the RNA null mutant. Our analysis thus reveals an unexpected role for oskar RNA during early oogenesis, independent of Oskar protein. These findings indicate that oskar RNA acts as a scaffold or regulatory RNA essential for development of the oocyte.