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.

中文翻译：

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驱动蛋白 1 非典型原肌球蛋白复合物转运 mRNA 的分子基础

Kinesin-1 在细胞内长距离携带货物，包括蛋白质、RNA、囊泡和病原体。驱动蛋白的机械化学循环已得到很好的描述，但它们如何建立货物特异性尚不完全清楚。将oskar mRNA运输到果蝇卵母细胞的后极是由果蝇驱动蛋白-1（也称为驱动蛋白重链 (Khc)）和推定的货物适配器、非典型原肌球蛋白Tm1介导的。这些蛋白质如何在 mRNA 运输中合作尚不清楚。在这里，我们展示了 Khc– Tm1复合物的高分辨率晶体结构。这些蛋白质形成一个三重盘绕线圈，包括两条注册 Khc 链和一条aTm1 链，反平行方向。我们表明Tm1与 Khc 上进化上保守的货物结合位点结合，突变分析证实了这种相互作用对体内 mRNA 转运的重要性。此外，我们证明 Khc 直接结合 RNA，并且它通过其替代货物结合域（与Tm1形成带正电的接合面）以及通过其相邻的辅助微管结合域实现。最后，我们表明Tm1在 Khc 与 RNA 的相互作用中起稳定作用，这将 Tm1 与经典电机适配器区分开来。