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Plant Signal Behav. 2014 May 15;9. pii: e29134. [Epub ahead of print]

Phosphothreonine 218 is required for the function of SR45.1 in regulating flower petal development in Arabidopsis.

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Department of Biology; Saint Bonaventure University; Saint Bonaventure, NY USA.
Department of Biology; Saint Bonaventure University; Saint Bonaventure, NY USA; Animal Biosciences and Biotechnology Laboratory; USDA-ARS; Beltsville, MD USA.
Soybean Genomics and Improvement Laboratory; USDA-ARS; Beltsville, MD USA.


RNA splicing is crucial to the production of mature mRNAs (mRNA). In Arabidopsis thaliana, the protein Arginine/Serine-rich 45 (SR45) acts as an RNA splicing activator and initiates the spliceosome assembly. SR45 is alternatively spliced into two isoforms. Isoform 1 (SR45.1) plays an important role in the flower petal development whereas isoform 2 (SR45.2) is important for root growth. In this study, we used immunoprecipitation to isolate an SR45.1-GFP fusion protein from transgenic plants complementing a null mutant, sr45-1. Mass spectrometry suggested a single phosphorylation event in a peptide from the alternatively spliced region unique to SR45.1. Substituting alanine for threonine 218, a candidate site for phosphorylation, did not complement the sr45-1 mutant with narrow flower petals whereas substituting aspartic acid or glutamic acid for threonine 218 did complement the sr45-1 mutant. Mass spectrometry also revealed that other proteins involved in the spliceosome co-precipitated with SR45.1, and RT-qPCR revealed that phosphorylation of threonine 218 promotes the function of SR45.1 in promoting the constitutive splicing of SR30 mRNA. This is the first demonstration of a specific phosphorylation site that differentially regulates the function of a plant splicing activator in physiologically and morphologically distinct plant tissues.


Co-immunoprecipitation; U5 small nuclear ribonucleoprotein; flower petal development; protein phosphorylation; regulation of RNA splicing


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