zation of Gag.L219A with SC35 and SF2 Phosphorylation is a major mechanism for regulating the localization of SR proteins in the nucleus, therefore we examined whether the degree of phosphorylation of splicing factors SC35 and SF2 influenced their association with Gag.L219A. To that end, murine Clk1, an SR protein kinase that phosphorylates the RS domains of SC35 and SF2 was expressed as an mCherry fusion protein in QT6 cells, either alone or in conjunction with Gag.L219A-CFP, SC35-YFP or YFP-SF2. When co-expressed with Gag.L219A, there was no significant co-localization with Clk1-mCherry Localization of Gag.L219A and nuclear body proteins in singly transfected QT6 cells. Co-localization analysis between Gag.L219A and the indicated nuclear body proteins in co-transfected QT6 cells. Merging of Gag.L219A and nuclear body marker protein channels is displayed. The DAPI channel is also depicted. The percentage of Gag.L219A with each factor is depicted in the Gag.L219A channel with the standard error of the mean. Scatterplot depicting the mean and standard error of the mean of Gag.L219A co-localization with each of the nuclear body protein. Still image from Supplemental Video 1 that closely examines a surface rendering of SC35 and Gag.L219A. Still image from Supplemental Video 2 that closely examines a surface rendering of SF2 and Gag.L219A. 6 Rice et al. Retroviral Gag and splicing factors row). However, as expected, there was co-localization between Clk1/SC35 and Clk1/SF2 because Clk1 phosphorylates both SC35 and SF2. In cells expressing Gag.L219A/Clk1/SC35, the degree of Gag co-localization with SC35 increased to 73.5% 5%, although the Chebulinic acid web increase was not statistically significant. However, Clk1 co-expression did significantly increase the co-localization of Gag.L219A with SF2 to 84% 2.2%. To assess whether Clk1 hyperphosphorylated YFP-SF2 and SC35-YFP in QT6 cells, we performed Western blotting of nuclear lysates. For both SF2 and SC35, there was a change in the migration of the hyperphosphorylated proteins in cells co-expressing Clk1-mCherry compared to the position of YFP-SF2 and SC35-YFP isolated from cells not expressing Clk1-mCherry. Treatment of the nuclear lysates with calf intestinal phosphatase dephosphorylated the proteins, as demonstrated by the faster migration of phosphatase-treated forms of SF2 and SC35, consistent with previous reports 7 Rice et al. Retroviral Gag and splicing factors . Together, these results suggest that the association of Gag.L219A with splicing factors, particularly SF2, PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19815280 was enhanced by hyperphosphorylation of the RS domain. The Number of Nuclear Gag.L219A Foci Increases with SC35 Overexpression During the course of our imaging studies, we noticed that the number of nuclear 
Gag.L219A-CFP foci appeared to increase in cells that co-expressed SC35-YFP. To determine whether this effect was specific for SC35, we compared the number of nuclear Gag.L219A foci in cells expressing Gag.L219A-CFP alone compared to cells transfected with equal amounts of pSC35-YFP, pYFP-SF2, or pYFP-PSP1. In cells expressing Gag.L219A alone, the average number of Gag foci was approximately 22 per nucleus, whereas upon co-expression of SC35-YFP, the average number of Gag nucleoplasmic foci increased significantly to 36. By contrast, co-transfection of equal amounts of pGag.L219A with pYFP-SF2 or pYFP-PSP1 did not lead to a significant change in the number of nuclear Gag foci. This experiment was repeated in DF1 cells with the same outc
