Directional Transport of Transcripts by Cytoplasmic Motor-Protein Complexes

在真核生物中，胞质信使核糖核酸的定向转运和转录产物的定位表达是实现细胞不对称的重要机制，是细胞分化和多细胞生物发育的先决条件。此外，神经元中的定向mRNA转运被用于突触前和突触后蛋白的表达，以产生神经元的可塑性。到目前为止，唯一全面鉴定的定向mRNA转运的例子是ASH1 mRNA在萌芽酵母中的移位。除了ASH1 mRNA，转运复合体由mRNA结合蛋白She2p组成，它与ASH1 mRNA中的四个元件结合，适配蛋白She3p和肌球蛋白V型马达蛋白Myo4p(图1A)。我们最近通过X射线结晶学表明She2p是一种新型的RNA结合蛋白(Nising等人，2004年)。通过使用X射线结晶学、生物物理、生化和活体技术的组合，我们现在的目标是在原子分辨率下表征依赖于马达蛋白的货物运输的分子基础。通过这样一种多学科的方法，我们试图了解ASH1 mRNP的核心因子是如何相互作用的：(I)检测它们的Cargo mRNA；(Ii)响应She2p对ASH1 mRNA的识别而组装成一个Cargo特异性的功能复合体；(Iii)通过细胞质转移ASH1 mRNA。

In eukaryotes, directional transport of cytoplasmic mRNA and subsequent localized expression of transcripts is a powerful mechanism to achieve cellular asymmetry, which is a prerequisite for cell differentiation and the development of multi-cellular organisms. In addition, directional mRNA-transport is utilized in neurons for pre- and post-synaptic protein expression to generate neuronal plasticity. Thus far, the only comprehensively characterized example for directional mRNA transport is the translocation of ASH1 mRNA in the budding yeast Saccharomyces cerevisiae.Besides ASH1 mRNA, the transport complex consists of the mRNA-binding protein She2p, which binds to four elements within the ASH1 mRNA, the adapter protein She3p, and the myosin type V motor protein Myo4p (Figure 1A). We recently showed by X-ray crystallography that She2p is a novel type of RNA-binding protein (Niessing et al., 2004). By using a combination of X-ray crystallography, biophysical, biochemical, and in vivo techniques, we now aim to characterize the molecular basis for motor-protein-dependent cargo transport at atomic resolution. With such a multidisciplinary approach, we attempt to understand how the core factors of the ASH1 mRNP interact to (i) detect their cargo mRNA, (ii) assemble into a cargo-specific, functional complex in response to ASH1 mRNA recognition by She2p, and (iii) translocate ASH1 mRNA through the cytoplasm.