Y14 is not methylated by PRMT5 but may modulate its methylation activity in vitro. A, GST-Y14/His-Magoh (YM, non-phosphorylated; pYM, phosphorylated) and GST-SmD1 were subjected to the in vitro methylation assay using recombinant GST-PRMT1 or immunopurified PRMT5 in the presence of 3H-labeled methyl donor and then detected by autoradiography (Auto-radio.) and Coomassie (Cooma.) Brilliant Blue staining. The lower panel shows Coomassie Blue staining of purified GST-PRMT1 (lane 1) and immunopurified FLAG-PRMT5 (lane 2). Lane 3 shows immunoblotting (IB) of the FLAG-PRMT5 co-precipitate using antibodies against PRMT5, pICln, and MEP50. The asterisk in lane 1 indicates a truncated Y14 fragment. B, in vitro methylation of GST-SmD1 using FLAG-PRMT5 immunoprecipitates was performed as in A; different amounts of GST-Y14/His-Magoh were used. Upper panel, autoradiogram; lower panel, Coomassie Blue staining.