TY - JOUR T1 - Two alternatively spliced isoforms of the Arabidopsis SR45 protein have distinct roles during normal plant development. JF - Plant Physiol Y1 - 2009 A1 - Zhang, Xiao-Ning A1 - Mount, Stephen M KW - Alternative Splicing KW - Amino Acid Sequence KW - Arabidopsis KW - Arabidopsis Proteins KW - Carrier Proteins KW - Flowers KW - Molecular Sequence Data KW - Mutation KW - Plant Roots KW - Protein Isoforms KW - Ribonucleoproteins KW - RNA-Binding Proteins KW - sequence alignment AB -

The serine-arginine-rich (SR) proteins constitute a conserved family of pre-mRNA splicing factors. In Arabidopsis (Arabidopsis thaliana), they are encoded by 19 genes, most of which are themselves alternatively spliced. In the case of SR45, the use of alternative 3' splice sites 21 nucleotides apart generates two alternatively spliced isoforms. Isoform 1 (SR45.1) has an insertion relative to isoform 2 (SR45.2) that replaces a single arginine with eight amino acids (TSPQRKTG). The biological implications of SR45 alternative splicing have been unclear. A previously described loss-of-function mutant affecting both isoforms, sr45-1, shows several developmental defects, including defects in petal development and root growth. We found that the SR45 promoter is highly active in regions with actively growing and dividing cells. We also tested the ability of each SR45 isoform to complement the sr45-1 mutant by overexpression of isoform-specific green fluorescent protein (GFP) fusion proteins. As expected, transgenic plants overexpressing either isoform displayed both nuclear speckles and GFP fluorescence throughout the nucleoplasm. We found that SR45.1-GFP complements the flower petal phenotype, but not the root growth phenotype. Conversely, SR45.2-GFP complements root growth but not floral morphology. Mutation of a predicted phosphorylation site within the alternatively spliced segment, SR45.1-S219A-GFP, does not affect complementation. However, a double mutation affecting both serine-219 and the adjacent threonine-218 (SR45.1-T218A + S219A-GFP) behaves like isoform 2, complementing the root but not the floral phenotype. In conclusion, our study provides evidence that the two alternatively spliced isoforms of SR45 have distinct biological functions.

VL - 150 CP - 3 M3 - 10.1104/pp.109.138180 ER - TY - JOUR T1 - The Drosophila U1-70K protein is required for viability, but its arginine-rich domain is dispensable. JF - Genetics Y1 - 2004 A1 - Salz, Helen K A1 - Mancebo, Ricardo S Y A1 - Nagengast, Alexis A A1 - Speck, Olga A1 - Psotka, Mitchell A1 - Mount, Stephen M KW - Amino Acid Sequence KW - Animals KW - Animals, Genetically Modified KW - Arginine KW - Drosophila KW - Drosophila Proteins KW - Molecular Sequence Data KW - Mutation KW - Protein Structure, Tertiary KW - Ribonucleoprotein, U1 Small Nuclear KW - RNA-Binding Proteins AB -

The conserved spliceosomal U1-70K protein is thought to play a key role in RNA splicing by linking the U1 snRNP particle to regulatory RNA-binding proteins. Although these protein interactions are mediated by repeating units rich in arginines and serines (RS domains) in vitro, tests of this domain's importance in intact multicellular organisms have not been carried out. Here we report a comprehensive genetic analysis of U1-70K function in Drosophila. Consistent with the idea that U1-70K is an essential splicing factor, we find that loss of U1-70K function results in lethality during embryogenesis. Surprisingly, and contrary to the current view of U1-70K function, animals carrying a mutant U1-70K protein lacking the arginine-rich domain, which includes two embedded sets of RS dipeptide repeats, have no discernible mutant phenotype. Through double-mutant studies, however, we show that the U1-70K RS domain deletion no longer supports viability when combined with a viable mutation in another U1 snRNP component. Together our studies demonstrate that while the protein interactions mediated by the U1-70K RS domain are not essential for viability, they nevertheless contribute to an essential U1 snRNP function.

VL - 168 CP - 4 M3 - 10.1534/genetics.104.032532 ER - TY - JOUR T1 - Sex-lethal splicing autoregulation in vivo: interactions between SEX-LETHAL, the U1 snRNP and U2AF underlie male exon skipping. JF - Development Y1 - 2003 A1 - Nagengast, Alexis A A1 - Stitzinger, Shane M A1 - Tseng, Chin-Hsiu A1 - Mount, Stephen M A1 - Salz, Helen K KW - Alternative Splicing KW - Amino Acid Sequence KW - Animals KW - Animals, Genetically Modified KW - Drosophila melanogaster KW - Drosophila Proteins KW - Exons KW - Female KW - Gene Expression Regulation, Developmental KW - Genes, Insect KW - Homeostasis KW - Male KW - Models, Genetic KW - Molecular Sequence Data KW - Nuclear Proteins KW - Point Mutation KW - Ribonucleoprotein, U1 Small Nuclear KW - Ribonucleoproteins KW - RNA Splicing KW - RNA-Binding Proteins KW - Sequence Homology, Amino Acid KW - Sex Differentiation AB -

Alternative splicing of the Sex-lethal pre-mRNA has long served as a model example of a regulated splicing event, yet the mechanism by which the female-specific SEX-LETHAL RNA-binding protein prevents inclusion of the translation-terminating male exon is not understood. Thus far, the only general splicing factor for which there is in vivo evidence for a regulatory role in the pathway leading to male-exon skipping is sans-fille (snf), a protein component of the spliceosomal U1 and U2 snRNPs. Its role, however, has remained enigmatic because of questions about whether SNF acts as part of an intact snRNP or a free protein. We provide evidence that SEX-LETHAL interacts with SANS-FILLE in the context of the U1 snRNP, through the characterization of a point mutation that interferes with both assembly into the U1 snRNP and complex formation with SEX-LETHAL. Moreover, we find that SEX-LETHAL associates with other integral U1 snRNP components, and we provide genetic evidence to support the biological relevance of these physical interactions. Similar genetic and biochemical approaches also link SEX-LETHAL with the heterodimeric splicing factor, U2AF. These studies point specifically to a mechanism by which SEX-LETHAL represses splicing by interacting with these key splicing factors at both ends of the regulated male exon. Moreover, because U2AF and the U1 snRNP are only associated transiently with the pre-mRNA during the course of spliceosome assembly, our studies are difficult to reconcile with the current model that proposes that the SEX-LETHAL blocks splicing at the second catalytic step, and instead argue that the SEX-LETHAL protein acts after splice site recognition, but before catalysis begins.

VL - 130 CP - 3 ER - TY - JOUR T1 - Genetic depletion reveals an essential role for an SR protein splicing factor in vertebrate cells. JF - Bioessays Y1 - 1997 A1 - Mount, S M KW - Amino Acid Sequence KW - Animals KW - Molecular Sequence Data KW - Nuclear Proteins KW - RNA Splicing KW - RNA-Binding Proteins KW - Serine-Arginine Splicing Factors KW - Vertebrates AB -

SR proteins are essential for the splicing of messenger RNA precursors in vitro, where they also alter splice site selection in a concentration-dependent manner. Although experiments involving overexpression or dominant mutations have confirmed that these proteins can influence RNA processing decisions in vivo, similar results with loss-of-function mutations have been lacking. Now, a system for genetic depletion of the chicken B cell line DT40 has revealed that the SR protein ASF/SF2 (alternative splicing factor/splicing factor 2) is essential for viability in these cells(1). This study opens the way for a complete functional dissection of this protein, and other SR proteins, in vivo.

VL - 19 CP - 3 M3 - 10.1002/bies.950190302 ER - TY - JOUR T1 - Genetic enhancement of RNA-processing defects by a dominant mutation in B52, the Drosophila gene for an SR protein splicing factor. JF - Mol Cell Biol Y1 - 1995 A1 - Peng, X A1 - Mount, S M KW - Alleles KW - Amino Acid Sequence KW - Animals KW - Base Sequence KW - DNA Primers KW - Drosophila melanogaster KW - Drosophila Proteins KW - Frameshift Mutation KW - Genes, Dominant KW - Genes, Insect KW - Molecular Sequence Data KW - Nuclear Proteins KW - Phosphoproteins KW - Point Mutation KW - Protein Structure, Tertiary KW - Proteins KW - RNA Splicing KW - RNA-Binding Proteins KW - Sequence Deletion KW - Sex Determination Analysis AB -

SR proteins are essential for pre-mRNA splicing in vitro, act early in the splicing pathway, and can influence alternative splice site choice. Here we describe the isolation of both dominant and loss-of-function alleles of B52, the gene for a Drosophila SR protein. The allele B52ED was identified as a dominant second-site enhancer of white-apricot (wa), a retrotransposon insertion in the second intron of the eye pigmentation gene white with a complex RNA-processing defect. B52ED also exaggerates the mutant phenotype of a distinct white allele carrying a 5' splice site mutation (wDR18), and alters the pattern of sex-specific splicing at doublesex under sensitized conditions, so that the male-specific splice is favored. In addition to being a dominant enhancer of these RNA-processing defects, B52ED is a recessive lethal allele that fails to complement other lethal alleles of B52. Comparison of B52ED with the B52+ allele from which it was derived revealed a single change in a conserved amino acid in the beta 4 strand of the first RNA-binding domain of B52, which suggests that altered RNA binding is responsible for the dominant phenotype. Reversion of the B52ED dominant allele with X rays led to the isolation of a B52 null allele. Together, these results indicate a critical role for the SR protein B52 in pre-mRNA splicing in vivo.

VL - 15 CP - 11 ER -