DYRK1A, but not cyclin E/CDK2 phosphorylates SF3b1 in vitro on phosphopeptides comigrating with the endogenous phosphopeptides from COS-7 cells. SF3b1-NT-His6 was labelled in vitro by GST-DYRK1A-ΔC (A) or cyclin E/CDK2 (B). GFP-SF3b1-NT was immunoprecipitated from COS-7 cells after metabolic labelling with 32PO4 (C). Recombinant proteins were subjected to two-dimensional phosphopeptide mapping. To verify that spots detected in different experiments are identical, samples were mixed and analysed on the same plate (A+B, A+C, B+C). Numbers label those spots that were both present in panels A and C. The panels show only the relevant area of the plates.