d At1g32490 codes for a DEAH box helicase. At5g08610 has been classified as RH26, and its phosphorylation sites are conserved in the most closely related member RH25 but not in the more distantly related RH31. At1g32490 shares high sequence homology with S.cerevisiae Prp22, Schizosaccharomyces pombe Cdc28/Prp8 and human DBP2, which are all DEAH box RNA helicases involved in pre-mRNA splicing. This suggests that the Nucleic Acids Research, 2006, Vol. 34, No. 11 3273 Arabidopsis homolog may function in splicing as well. Although MedChemExpress Regadenoson highly similar in their C-terminal regions, the Ntermini of At1g32490, HsDBP2 and SpCdc28/Prp8 are not very well conserved. Nevertheless, both HsDBP2 and SpCdc28/Prp8 contain one Ser residue at the corresponding position, which are followed by an acidic residue as well. Three phosphorylation sites of the two Arabidopsis RNA helicases are followed by an Asp and all sites are located in the N-terminus of the proteins. Besides the DEAD box helicase, only 2 of the 22 phosphoproteins predicted to function in RNA metabolism could not be directly related to RNA splicing. The first is encoded by At5g48650, and this protein contains a nuclear transport factor 2 -like domain and an RRM domain. Its structure suggests that the protein may be involved in the nucleocytoplasmic transport of RNA or RNA-containing complexes. Arabidopsis At5g48650 has a similar domain organization as S.cerevisiae Bre5 that acts as a cofactor for the Ubp3 de-ubiquitinating enzyme. Structurally similar mammalian proteins are Ras-GTPaseactivating protein SH3 domain-binding proteins. A closer inspection of the polypeptide sequence showed that the Arabidopsis protein, like HsG3BP1, also has a potential SH3 domain-binding site and a C-terminal RGG/ glycine-rich domain, but lacks the acidic domain. Human G3BPs are implicated in signal transduction downstream of Ras and may act as endonucleases. Phosphorylation of HsG3BP1 at PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19816862 Ser-149 plays a role in regulation of its endonuclease activity and subcellular localization. HsG3BP1 is phosphorylated at a second site, Ser-232, which is followed by a Pro. Interestingly, the determined site of the Arabidopsis homolog is followed by a Pro as well, and is located at a similar position. Therefore it is tempting to speculate that similar kinases may target these homologs in both plants and mammals. The nature of the kinase targeting Ser-232 in HsG3BP1 and the functional effect of this phosphorylation event is unknown. Several proteins with the same domain organization are encoded by the Arabidopsis genome, but the region containing the phosphorylation site that we determined is not conserved in these homologs. The second protein contains an RRM domain and its C-terminal phosphorylation site is conserved in homologs of other dicot species, but absent in monocot homologs. Arabidopsis contains a closely related protein that is 78% identical to At4g17720. The phosphorylation site is conserved in this homolog. In conclusion, our analysis shows that many Arabidopsis SR proteins are phosphoproteins and that they are almost exclusively phosphorylated in their RS domains. In total, 50 of the 79 phosphorylation sites were on 
SR proteins, 18 sites on other RS domain-containing proteins and the remaining 11 sites on proteins lacking RS domains. Phosphorylation of alternative splice isoforms Different isoforms of many plant SR proteins and other RS domain-containing proteins are generated by alternative 3274 Nucleic Acids Research,
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