Trends in Cell Biology
Why do cells need an assembly machine for RNA–protein complexes?
Section snippets
The SMN complex
Important and unexpected insights into the process of snRNP assembly came from studies on the function of the survival of motor neurons (SMN) protein 24, 25, 26, 27. Reduced levels of SMN, due to a genetic defect, cause degeneration of motor neurons in the spinal cord and result in spinal muscular atrophy (SMA) 28, 29. The SMN protein is expressed in all eukaryotes tested so far, except Saccharomyces cerevisiae, and in all cell types of vertebrate organisms. Particularly high levels of SMN are
The SMN-dependent assembly of Sm cores
Experiments in Xenopus oocytes and mammalian somatic cells revealed an essential role for the SMN complex in spliceosomal snRNP assembly 25, 31, 38, 47. Further evidence that the SMN complex is needed for assembly of both Sm-site-containing snRNPs and the mixed Sm- and Lsm (Sm-like)-containing core found in U7 snRNP was provided using cell extracts and purified components 36, 48, 49, 50. The SMN-dependent assembly of Sm cores in cell extracts requires ATP hydrolysis 36, 48, 49. Depletion of the
Spliceosomal snRNAs contain high-affinity binding domains for the SMN complex
There are other Sm-site-containing spliceosomal snRNAs that do not contain the U1 SL1 sequence, and yet SMN mediates their assembly with Sm proteins [36]. The SMN complex binds directly to all Sm-site-containing major spliceosomal snRNAs, and delineation of the sequence elements of the snRNAs responsible for SMN-complex binding has revealed that, unlike for U1 snRNA, the SMN complex binds to U2, U4 and U5 snRNAs through domains near their 3′-ends. All of these snRNAs contain at least one
The SMN complex determines the specificity of snRNP assembly
The SMN complex binds directly and sequence-specifically to SL1 of U1 snRNA. Mutations in the SL1 sequence that impair this binding also impair the assembly of U1 snRNP in vitro and in Xenopus oocytes [37]. The sequence-specific binding of the SMN complex to a specific snRNA is crucial for selection of the snRNA as a target to ensure Sm-core assembly only on targeted RNA, thus preventing promiscuous and deleterious binding of Sm proteins to other RNAs [36]. Although an Sm site and a
The SMN complex in the metabolism and assembly of diverse RNPs
Various studies have suggested that the SMN complex also plays a role in the assembly and metabolism of other types of RNPs, including small nucleolar RNPs (snoRNPs), heterogeneous nuclear RNPs, the complexes that carry out transcription and pre-mRNA splicing and, possibly, micro RNPs 26, 37, 39, 47, 48, 49, 50, 51, 58, 59, 60, 61. It is likely that, given the numerous RNA-binding proteins with which the SMN complex interacts, there are several RNA targets for it in cells, and that it is also
Concluding remarks
Because RNA-binding proteins interact readily with their cognate RNAs, and because specific RNP structures such as Sm cores can form even on minimal RNAs, there does not seem to be a reason for cells to need a specialized molecular machine to govern their associations. Perhaps the key to understanding the essential function of the SMN complex in these assembly processes is the fact that such RNP-assembly reactions can occur so readily. In the case of the spliceosomal snRNPs, the Sm proteins
Acknowledgements
We thank the members of our laboratory, especially Amelie Gubitz and Daniel Battle, for helpful discussions and comments on this manuscript. The work in our laboratory reported here was supported by the Association Française Contre les Myopathies and by a grant from the NIH. G.D. is an Investigator of the Howard Hughes Medical Institute.
References (62)
Structure of spliceosomal snRNPs and their role in pre-mRNA splicing
Biochim. Biophys. Acta
(1990)- et al.
Spliceosomal UsnRNP biogenesis, structure and function
Curr. Opin. Cell Biol.
(2001) Crystal structures of two Sm protein complexes and their implications for the assembly of the spliceosomal snRNPs
Cell
(1999)PHAX, a mediator of U snRNA nuclear export whose activity is regulated by phosphorylation
Cell
(2000)Cytoplasmic assembly of snRNP particles from stored proteins and newly transcribed snRNAs in L929 mouse fibroblasts
Exp. Cell Res.
(1988)Newly synthesized small nuclear RNAs appear transiently in the cytoplasm
J. Mol. Biol.
(1988)Cap trimethylation of U snRNA is cytoplasmic and dependent on U snRNP protein binding
Cell
(1986)The trimethylguanosine cap structure of U1 snRNA is a component of a bipartite nuclear targeting signal
Cell
(1990)The SMN–SIP1 complex has an essential role in spliceosomal snRNP biogenesis
Cell
(1997)The spinal muscular atrophy disease gene product, SMN, and its associated protein SIP1 are in a complex with spliceosomal snRNP proteins
Cell
(1997)