A 586 aa Fragment of HTT Is SUMO Modified Downstream of Exon 1.Figure 5(A) SUMOplot (Abgent) predicts that HTT is SUMOylated downstream of HTTex1p lysines. SUMOplot analysis predicts two high-probability SUMO sites (red) and three low-probability SUMO sites (blue) identified downstream of HTT exon 1. Greater than 13 overlapping noncovalent SIMs are observed within this fragment (yellow). (B) Longer HTT polypeptides are modified by SUMO-1. Western blot overexpressing HTTex1p (46QP-H4 or 3R), unexpanded HTT-586 fragment (25Q-586 or 25Q-3R-586 aa), and expanded HTT-586 fragment (137Q-586 or 137Q-3R-586 aa) with SUMO-1 (GFP-SUMO-1). Cell lysates were subjected to HTT immunoprecipitation (IP) using HTT antibody. Western analysis performed with the Odyssey (LI-COR) allows detection of HTT (data not shown) and SUMO simultaneously and shows that all forms of HTT are SUMO-1 modified using the anti-GFP antibody. HTTex1p is covalently SUMO-1 modified (lane 3), and the modification disappears when Lys are mutated to Arg (3R) (lane 4). Both unexpanded (lane 4 and 5) and expanded HTT-586 fragments (lane 6 and 7) are covalently SUMO-1 modified in both the presence and absence of the three Lys in the N-terminal region of HTT (3R). Free SUMO-1 is indicated with the arrow, and SUMO-modified HTT is indicated by the boxes. Inset on the right, from a replicate experiment, shows comigration of expanded HTT (anti-HTT) and SUMO-1 (anti-GFP) displayed in the gray scale and in color when the two antibodies are merged (HTT in red, SUMO-1 in green, and yellow when colocalizing). (C) Bait plasmids (HTT-586-25Q or HTT-586-73Q aa) were transformed into the L40ccua MATa yeast strain. Yeast clones encoding bait proteins were individually mated against a matrix of MATα yeast clones encoding 16,888 prey proteins (with Gal4 activation domain fusions) using pipetting and spotting robots. Diploid yeasts were spotted onto SDIV (-Leu-Trp-Ura-His) agar plates for selection of PPIs as well as nylon membranes placed on SDIV agar plates for β-galactosidase assays. After 5-6 days of incubation at 30ºC, digitized images of the agar plates and nylon membranes were assessed for growth and β-galactosidase activity using the software Visual Grid (GPC Biotech). (D) Overexpression of PIAS1 alone, with unexpanded HTT constructs (25Q-586 aa) plus SUMO-1 (GFP-SUMO-1), shows that PIAS1 binds both full-length HTT and HTT-586 fragment (25Q-586 aa). WT HTT was used in these experiments to preclude confounding aggregation effects. Western analysis detection was performed using Odyssey and is displayed in the gray scale but is shown in color on the merge (HTT is red, and PIAS1 is green). WB, western blot. (E) HeLa cells overexpressing either expanded 586 aa-HTT or the phosphomimetic (S13,16D = DD) with SUMO-2 plus and minus PIAS1. HTT was purified by IP using hydrazide beads (Bioclone) crosslinked to HTT (Enzo) antibody and subjected to western analysis using anti-HTT (MAB5490). Arrows indicate SUMO-2-modified HTT. (F) IP of HTT with hydrazide-linked beads shows that both unexpanded and expanded HTT 586 aa phosphomimetics (S13, 16D-586 aa) are modified by SUMO-2. Arrows indicate SUMO-conjugated HTT. Note that all experiments including the Y2H assay were done in triplicate; representative experiments are shown. Arrows indicate SUMO-2-modified HTT.