Nucleoprotein assemblies at the nanoscale: medical implications

Bionanotechnology is exploiting the rich structural knowledge now available on DNA and DNA–protein interactions to construct nucleoprotein-based devices that have the potential not only to contribute to our understanding of the structure and function of the proteins and nucleic acids involved but also to new approaches to problems in medicine. Assemblies under development currently are poised to contribute to diagnosis and therapy. Here, I discuss recent work in this emerging field. A key turning point in the evolution of life on Earth appears to have occurred at the transition from the RNA world [1] to the ribonucleoprotein world, prior to the evolution of the early nucleoprotein world of proteins, nucleic acids and their products. At that juncture, protein synthesis appeared and began to modify and facilitate the action of the ribozymes that had previously carried out the functions of living systems alone. Telomerase [2] and the ribosome [3] are the best understood of these ancient nanoscale nucleoprotein assemblies, since they appear to have survived as molecular fossils. The wealth of diversity that has emerged since this shift in information transfer occurred is virtually unlimited, with the modern chromosome being perhaps the most elaborate nucleoprotein assembly that has emerged thus far [4,5]. In the strictest sense, these entities are naturally occurring nanodevices that can serve as structural models in nanotechnology. Bionanotechnology is rapidly exploiting the available structural knowledge on nucleoprotein assemblies of this kind and may soon develop novel assemblies that will not only have medical applications, but will also contribute to our general undersanding of the structure and function of these same assemblies. This review discusses recent work in this area. Nucleic acid subassemblies The structures of telomerase and the ribosome suggest that stable nucleoprotein assemblies can be assembled on complex RNA substructures. Rational design of such structures has not been explored extensively since the rules for RNA folding are not yet well understood and because RNA is not particularly stable. Although aptamers of RNA can be selected so as to conform to almost any available binding surface, this property of RNA has not yet been exploited extensively in the construction of nucleoprotein-based nanodevices. This may be due to the relatively high dissociation constants observed for aptamer–protein complexes [6,7] and aptamer–ligand complexes compared with their protein analogs [8] or to the relatively time-consuming processes involved in aptamer selection and characterization [9,10]. However, duplex DNA can now be molded into a nearly endless variety of shapes with relative ease. Carefully chosen complementary oligodeoxynucleotides can be synthesized easily and allowed to self-assemble under mild conditions. The substructures thus formed can be ligated into permanently locked conformations [11] or, in most cases, are simply stable without further manipulation [12]. These facile techniques have yielded duplex DNA substructures with the connectivity of the platonic solids [13–15], Borromean rings [16] and model recombination intermediates with multiple arms and interconnections [17–19]. The recombination intermediates have been particularly useful in the formation of extended 2D tilings [19–21], while more complex approaches that use partially single-stranded assemblies are able to generate almost any 2D shape [22]. Assembly of nucleoprotein devices Given the rich collection of well characterized self-assembling DNA structures and the elaborate approaches to the design of new assemblies, it is not surprising that DNA has been the most extensively exploited of the nucleic acids in the bionanotechnology of nucleoprotein assembly. An essential step in the construction of these nucleoprotein assemblies is selective coupling of