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标题：Hooking She3p onto She2p for myosin-mediated cytoplasmic mRNA transport
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作者：Nimisha Singh ; Günter Blobel ; Hang Shi 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：1
页码：142-147
DOI：10.1073/pnas.1423194112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceAfter export from the nucleus, distinct mRNAs are further transported to specific locales in the cytoplasm. In yeast, a subset of mRNAs targeted to the daughter cells carry zipcodes, which are recognized by two distinct binding proteins, one of which also binds to myosin. The assembled myosin motor walks with its mRNA cargo, like a ropewalker, on an actin filament. To avoid futile movement, motor assembly is strictly controlled. We show that a striking hook-like structure of one zipcode-binding protein connects to a pocket of the other zipcode-binding protein. Hooking is proposed to align the zipcode-binding sites of the two proteins to create a composite site for high-affinity zipcode binding. The segregation of approximately two dozen distinct mRNAs from yeast mother to daughter cell cytoplasm is a classical paradigm for eukaryotic mRNA transport. The information for transport resides in an mRNA element 40-100 nt in length, known as "zipcode." Targeted transport requires properly positioned actin filaments and cooperative loading of mRNA cargo to myosin. Cargo loading to myosin uses myosin 4 protein (Myo4p), swi5p-dependent HO expression 2 protein (She2p) and 3 protein (She3p), and zipcode. We previously determined a crystal structure of Myo4p and She3p, their 1:2 stoichiometry and interactome; we furthermore showed that the motor complex assembly requires two Myo4p*She3p heterotrimers, one She2p tetramer, and at least a single zipcode to yield a stable complex of [Myo4p*She3p*She2p*zipcode] in 2:4:4:1 stoichiometry in vitro. Here, we report a structure at 2.8-[IMG]f1.gif" ALT="A" BORDER="0"> resolution of a cocrystal of a She2p tetramer bound to a segment of She3p. In this crystal structure, the She3p segment forms a striking hook that binds to a shallow hydrophobic pocket on the surface of each She2p subunit of the tetramer. Both She3p hook and cognate She2p binding pocket are composed of highly conserved residues. We also discovered a highly conserved region of She3p upstream of its hook region. Because this region consists of basic and aromatic residues, it likely represents part of She3p's binding activity for zipcode. Because She2p also exhibits zipcode-binding activity, we suggest that "hooking" She3p onto She2p aligns each of their zipcode-binding activities into a high-affinity site, thereby linking motor assembly to zipcode.
关键词：crystal structure ; LPGV hook ; type II turn ; motor assembly