In Vitro Reconstitution of Kinesin-Based, Axonal mRNA Transport

Motor protein-driven transport of mRNAs on microtubules and their local translation underlie important neuronal functions such as development, growth cone steering, and synaptic plasticity. While there is abundant data on how membrane-bound cargoes such as vesicles, endosomes, or mitochondria are coupled to motor proteins, surprisingly little is known on the direct interactions of RNA-protein complexes and kinesins or dynein. Provided the potential building blocks are identified, in vitro reconstitutions coupled to Total Internal Reflection Microscopy (TIRF-M) are a powerful and highly sensitive tool to understand how single molecules dynamically interact to assemble into functional complexes. Here we describe how we assemble TIRF-M imaging chambers suitable for the imaging of single protein-RNA complexes. We give advice on optimal sample preparation procedures and explain how a minimal axonal mRNA transport complex can be assembled in vitro. As these assays work at picomolar-range concentrations of proteins and RNAs, they allow the investigation of molecules that cannot be obtained at high concentrations, such as many large or disordered proteins. This now opens the possibility to study how RNA-binding proteins (RBPs), RNAs, and microtubule-associated proteins act together in real-time at single-molecule sensitivity to create cytoplasmic mRNA distributions.