From Virus Structure to Spliceosome Function Via RNA Splicing

When splicing was discovered, major effort was put into developing simple test-tube systems to study the splicing reaction. These studies led to elucidation of the chemistry of the splicing reaction, which occurs when the pre-mRNA is assembled in a large complex of RNA and proteins termed spliceosome. Recent revolution in electron microscopy (EM) in the frozen-hydrated state (cryo-EM) led to atomic structures of a number of spliceosome intermediates. Most mammalian pre-mRNAs have many introns. Thus, alternative splicing - whereby different combinations of exons are spliced together to produce different mRNAs from a single copy gene - is a major source for the diversity of the human proteome. Notably, the accurate processes of RNA splicing and alternative splicing are crucial regulators of gene expression and cell function, and defects in them underlie many human diseases, including cancer. Several groups around the world studied this novel phenomenon of splicing, and I’ll discuss the contribution of Israeli scientists to this important issue (Aloni et al 1977; Lavi & Groner 1977). Splicing was discovered when I was doing my research on gene expression. Since my post-doctoral studies at MRC-LMB in Cambridge/UK with Sir Aaron Klug, a pioneer of structural biology and a 1982 sole Nobel Laureate, I was interested in biological assembly systems of proteins and nucleic acids. Enthusiastic of the discovery of splicing and its biological significance, I shifted fields from chromatin to splicing. Together with my partner Prof Yossi Sperling from the Weizmann Institute we took a different approach and collaborated on the study of the structure and function of the natural splicing complex – the endogenous spliceosome - that we isolated from cell nuclei under native conditions. The endogenous spliceosome is larger than the spliceosome assembled in the test-tube (which most other teams chose to focus on).

Structural studies by EM and EM in the frozen-hydrated state, combined with molecular biology, revealed that the endogenous spliceosome is composed of four native spliceosomes, each similar to the test-tube spliceosome, which are connected by the pre-mRNA, hence termed supraspliceosome (Sperling et al 2008; R. Sperling 2017). Importantly, the entire repertoire of nuclear pre-mRNAs is assembled in splicing-active supraspliceosomes. A remarkable feature of supraspliceosomes is that they individually package a single pre-mRNA transcript of different size and number of introns into complexes of a unique structure, indicating their universal nature. This multi-subunit complex also regulates alternative splicing, and harbors components of all the pre-mRNA processing activities. Thus, the supraspliceosome is a stand-alone complete macromolecular machine capable of performing splicing, alternative splicing, and encompass all the nuclear processing activities that the pre-mRNA has to undergo before it can exit from the nucleus to the cytoplasm to encode for proteins.