SF3b1 is phosphorylated at Thr434 by endogenous kinases in COS-7 cells and in vitro by DYRK1A. A, SF3b1-NT-His6 (wt) and point mutants of Thr273 (T273A) or Thr434 (T434A) were phosphorylated by GST-DYRK1A-ΔC in vitro. B, GFP-SF3b1-NT (wt) or the alanine mutant of Thr434 (T434A) were expressed in COS-7 cells, metabolically labelled with 32P and purified by immunoprecipitation with a GFP-specific antiserum. Phosphopeptide maps were generated as described above. Arrows point to phosphopeptides that are absent in the mutants. X marks a spot apparently superimposing spot1 (see text).