Gideon Dreyfuss

Messenger-RNA-binding proteins and the messages they carry

From sites of transcription in the nucleus to the outreaches of the cytoplasm, messenger RNAs are associated with RNA-binding proteins. These proteins influence pre-mRNA processing as well as the transport, localization, translation and stability of mRNAs. Recent discoveries have shown that one group of these proteins marks exon?exon junctions and has a role in mRNA export. These proteins communicate crucial information to the translation machinery for the surveillance of nonsense mutations and for mRNA localization and translation.

RNA and Disease

Cellular functions depend on numerous protein-coding and noncoding RNAs and the RNA-binding proteins associated with them, which form ribonucleoprotein complexes (RNPs). Mutations that disrupt either the RNA or protein components of RNPs or the factors required for their assembly can be deleterious. Alternative splicing provides cells with an exquisite capacity to fine-tune their transcriptome and proteome in response to cues. Splicing depends on a complex code, numerous RNA-binding proteins, and an enormously intricate network of interactions among them, increasing the opportunity for exposure to mutations and misregulation that cause disease. The discovery of disease-causing mutations in RNAs is yielding a wealth of new therapeutic targets, and the growing understanding of RNA biology and chemistry is providing new RNA-based tools for developing therapeutics.

Transport of proteins and RNAs in and out of the nucleus.

We are most grateful to Dr. Ueli Aebi and Dr. Brian Burke for the drawing of the NPC on which Figure 2Figure 2 is based, and to an anonymous reviewer who provided suggestions for condensing and improving the manuscript. We hope colleagues will accept our sincere apologies for omitting, due to space limitations, many citations of very important contributions to the field. Work on nuclear transport in this laboratory was supported by grants from the National Institutes of Health and the Human Frontier Science Program Organization. Gideon Dreyfuss is an Investigator of the Howard Hughes Medical Institute.

A NOVEL NUCLEAR STRUCTURE CONTAINING THE SURVIVAL OF MOTOR NEURONS PROTEIN

Spinal muscular atrophy (SMA) is a common, often fatal, autosomal recessive disease leading to progressive muscle wasting and paralysis as a result of degeneration of anterior horn cells of the spinal cord. A gene termed survival of motor neurons (SMN), at 5q13, has been identified as the determining gene of SMA (Lefebvre et al., 1995). The SMN gene is deleted in > 98% of SMA patients, but the function of the SMN protein is unknown. In searching for hnRNP‐interacting proteins we found that SMN interacts with the RGG box region of hnRNP U, with itself, with fibrillarin and with several novel proteins. We have produced monoclonal antibodies to the SMN protein, and we report here on its striking cellular localization pattern. Immunolocalization studies using SMN monoclonal antibodies show several intense dots in HeLa cell nuclei. These structures are similar in number (2–6) and size (0.1–1.0 micron) to coiled bodies, and frequently are found near or associated with coiled bodies. We term these prominent nuclear structures gems, for Gemini of coiled bodies.

RNA-binding proteins and post-transcriptional gene regulation.

RNAs in cells are associated with RNA‐binding proteins (RBPs) to form ribonucleoprotein (RNP) complexes. The RBPs influence the structure and interactions of the RNAs and play critical roles in their biogenesis, stability, function, transport and cellular localization. Eukaryotic cells encode a large number of RBPs (thousands in vertebrates), each of which has unique RNA‐binding activity and protein–protein interaction characteristics. The remarkable diversity of RBPs, which appears to have increased during evolution in parallel to the increase in the number of introns, allows eukaryotic cells to utilize them in an enormous array of combinations giving rise to a unique RNP for each RNA. In this short review, we focus on the RBPs that interact with pre‐mRNAs and mRNAs and discuss their roles in the regulation of post‐transcriptional gene expression.

A Novel Receptor-Mediated Nuclear Protein Import Pathway

Targeting of most nuclear proteins to the cell nucleus is initiated by interaction between the classical nuclear localization signals (NLSs) contained within them and the importin NLS receptor complex. We have recently delineated a novel 38 amino acid transport signal in the hnRNP A1 protein, termed M9, which confers bidirectional transport across the nuclear envelope. We show here that M9-mediated nuclear import occurs by a novel pathway that is independent of the well-characterized, importin-mediated classical NLS pathway. Additionally, we have identified a specific M9-interacting protein, termed transportin, which binds to wild-type M9 but not to transport-defective M9 mutants. Transportin is a 90 kDa protein, distantly related to importin beta, and we show that it mediates the nuclear import of M9-containing proteins. These findings demonstrate that there are at least two receptor-mediated nuclear protein import pathways. Furthermore, as hnRNP A1 likely participates in mRNA export, it raises the possibility that transportin is a mediator of this process as well.

The SMN-SIP1 Complex Has an Essential Role in Spliceosomal snRNP Biogenesis

Spinal muscular atrophy (SMA) is an often fatal neuromuscular disease that has been directly linked to the protein product of the Survival of Motor Neurons (SMN) gene. The SMN protein is tightly associated with a novel protein, SIP1, and together they form a complex with several spliceosomal snRNP proteins. Here we show that the SMN-SIP1 complex is associated with spliceosomal snRNAs U1 and U5 in the cytoplasm of Xenopus oocytes. Antibodies directed against the SMN-SIP1 complex strongly interfere with the cytoplasmic assembly of the common (Sm) snRNP proteins with spliceosomal snRNAs and with the import of the snRNP complex into the nucleus. Thus, the SMN-SIP1 complex is directly involved in the biogenesis of spliceosomal snRNPs. Defects in spliceosomal snRNP biogenesis may, therefore, be the cause of SMA.

The Spinal Muscular Atrophy Disease Gene Product, SMN, and Its Associated Protein SIP1 Are in a Complex with Spliceosomal snRNP Proteins

Spinal muscular atrophy (SMA), one of the most common fatal autosomal recessive diseases, is characterized by degeneration of motor neurons and muscular atrophy. The SMA disease gene, termed Survival of Motor Neurons (SMN), is deleted or mutated in over 98% of SMA patients. The function of the SMN protein is unknown. We found that SMN is tightly associated with a novel protein, SIP1, and together they form a specific complex with several spliceosomal snRNP proteins. SMN interacts directly with several of the snRNP Sm core proteins, including B, D1-3, and E. Interestingly, SIP1 has significant sequence similarity with Brr1, a yeast protein critical for snRNP biogenesis. These findings suggest a role for SMN and SIP1 in spliceosomal snRNP biogenesis and function and provide a likely molecular mechanism for the cause of SMA.

The protein product of the fragile X gene, FMR1, has characteristics of an RNA-binding protein

Fragile X syndrome is one of the most common human genetic diseases and the most common cause of hereditary mental retardation. The gene that causes fragile X syndrome, FMR1, was recently identified and sequenced and found to encode a putative protein of unknown function. Here we report that FMR1 contains two types of sequence motifs recently found in RNA-binding proteins: an RGG box and two heterogeneous nuclear RNP K homology domains. We also demonstrate that FMR1 binds RNA in vitro. Using antibodies to FMR1, we detect its expression in divergent organisms and in cells of unaffected humans, but fragile X-affected patients express little or no FMR1. These findings demonstrate that FMR1 expression is directly correlated with the fragile X syndrome and suggest that anti-FMR1 antibodies will be important for diagnosis of fragile X syndrome. Furthermore, the RNA binding activity of FMR1 opens the way to understanding the function of FMR1.

A nuclear export signal in hnRNP A1: A signal-mediated, temperature-dependent nuclear protein export pathway

Pre-mRNAs are associated with hnRNPs, and these proteins play important roles in the biogenesis of mRNAs. The hnRNP A1 is one of the most abundant hnRNPs, and although localized primarily in the nucleoplasm, shuttles continuously between the nucleus and the cytoplasm. A 38 amino acid domain within A1, termed M9, which bears no resemblance to classical nuclear localization signal (NLS) sequences, localizes A1 to the nucleus. Here we show that M9 is also a nuclear export signal; placing M9 on a protein that is otherwise restricted to the nucleus, the nucleoplasmin core domain (NPc), efficiently exports it to the cytoplasm in a temperature-dependent manner. In contrast, classical NLSs cannot promote the export of NPc. These findings demonstrate that there is a signal-dependent, temperature-sensitive nuclear export pathway and strengthen the suggestion that A1 and other shuttling hnRNPs function as carriers for RNA during export to the cytoplasm.