The Cajal body: a meeting place for spliceosomal snRNPs in the nuclear maze

Abstract

Spliceosomal small nuclear ribonucleoprotein particles (snRNPs) are essential pre-mRNA splicing factors that consist of small nuclear RNAs (snRNAs) complexed with specific sets of proteins. A considerable body of evidence has established that snRNP assembly is accomplished after snRNA synthesis in the nucleus through a series of steps involving cytoplasmic and nuclear phases. Recent work indicates that snRNPs transiently localize to the Cajal body (CB), a nonmembrane-bound inclusion present in the nuclei of most cells, for the final steps in snRNP maturation, including snRNA base modification, U4/U6 snRNA annealing, and snRNA-protein assembly. Here, we review these findings that suggest a crucial role for CBs in the spliceosome cycle in which production of new snRNPs--and perhaps regenerated snRNPs after splicing--is promoted by the concentration of substrates in this previously mysterious subnuclear organelle. These insights allow us to speculate on the role of nuclear bodies in regulating the dynamics of RNP assembly to maintain a functional pool of factors available for key steps in gene expression.

Coilin-dependent snRNP assembly is essential for zebrafish embryogenesis

“…We attribute the lethality of coilin depletion to defects in late stages of snRNP biogenesis. After core snRNPs are reimported to the nucleus and snRNAs are modified, final steps of maturation must occur before snRNPs are competent for splicing; these include RNA structural rearrangements and recruitment of additional snRNPspecific proteins 5 . For example, the U4/U6•U5 tri-snRNP is formed from three individual snRNPs (U4, U5 and U6) that are inactive for splicing before assembly.…”

“…Similarly to the tri-snRNP, intermediates in mature U2 snRNP formation can be trapped in CBs upon knockdown of appropriate assembly factors 17 . U2 snRNP is present in cells in 12S and 17S forms, reflecting the association of the core U2 snRNP containing the Sm ring U2A and U2B proteins (12S) with the SF3 complex (17S) 5,6 . Notably, both the 12S and 17S forms of the U2 snRNP rescued coilin morphants equally, indicating that formation of the 12S U2 snRNP is compromised upon coilin depletion.…”

“…However, coilin has never been identified as a stable component of any of the macromolecular complexes found in CBs, such as the spliceosomal snRNPs, despite exhaustive biochemical study and characterization by MS 4 . Taken together, this evidence has led to the view that coilin provides the structural scaffold of CBs through self association and a network of low-affinity interactions with CB components 1,5,24,28 .…”

“…Yet CB components exchange rapidly with the nucleoplasm, where most of these processes occur 3 . The challenge, therefore, has been to identify the function of CBs.Despite the localization of splicing machinery in CBs, they are not the sites of pre-mRNA splicing 4,5 . Splicing occurs throughout the nucleus and is catalyzed by the spliceosome, a macromolecular complex formed on pre-mRNA from five essential snRNPs 6 .…”

“…However, coilin has never been identified as a stable component of any of the macromolecular complexes found in CBs, such as the spliceosomal snRNPs, despite exhaustive biochemical study and characterization by MS 4 . Taken together, this evidence has led to the view that coilin provides the structural scaffold of CBs through self association and a network of low-affinity interactions with CB components 1,5,24,28 .Coilin clearly has a central role in the integrity of the CB, but the functional consequences to cellular or organismal physiology from Spliceosomal small nuclear ribonucleoproteins (snRNPs), comprised of small nuclear RNAs (snRNAs) in complex with snRNPspecific proteins, are essential for pre-mRNA splicing. Coilin is not a snRNP protein but concentrates snRNPs and their assembly intermediates in Cajal bodies (CBs).…”