Engineering Crystal Packing in RNA-Protein Complexes II: A Historical Perspective from the Structural Studies of the Spliceosome

Structural studies of the spliceosome: Bridging the gaps

The spliceosome is a multi-megadalton RNA-protein complex responsible for the removal of non-coding introns from pre-mRNAs. Due to its complexity and dynamic nature, it has proven to be a very challenging target for structural studies. Developments in single particle cryo-EM have overcome these previous limitations and paved the way towards a structural characterisation of the splicing machinery. Despite tremendous progress, many aspects of spliceosome structure and function remain elusive. In particular, the events leading to the definition of exon-intron boundaries, alternative and non-canonical splicing events, and cross-talk with other cellular machineries. Efforts are being made to address these knowledge gaps and further our mechanistic understanding of the spliceosome. Here, we summarise recent progress in the structural and functional analysis of the spliceosome.

Engineering Crystal Packing in RNA Structures I: Past and Future Strategies for Engineering RNA Packing in Crystals

X-ray crystallography remains a powerful method to gain atomistic insights into the catalytic and regulatory functions of RNA molecules. However, the technique requires the preparation of diffraction-quality crystals. This is often a resource- and time-consuming venture because RNA crystallization is hindered by the conformational heterogeneity of RNA, as well as the limited opportunities for stereospecific intermolecular interactions between RNA molecules. The limited success at crystallization explains in part the smaller number of RNA-only structures in the Protein Data Bank. Several approaches have been developed to aid the formation of well-ordered RNA crystals. The majority of these are construct-engineering techniques that aim to introduce crystal contacts to favor the formation of well-diffracting crystals. A typical example is the insertion of tetraloop-tetraloop receptor pairs into non-essential RNA segments to promote intermolecular association. Other methods of promoting crystallization involve chaperones and crystallization-friendly molecules that increase RNA stability and improve crystal packing. In this review, we discuss the various techniques that have been successfully used to facilitate crystal packing of RNA molecules, recent advances in construct engineering, and directions for future research in this vital aspect of RNA crystallography.