Are STATS arginine-methylated?

Transcription factors of the STAT (signal transducer and activator of transcription) family are important in signal transduction of cytokines. They are subject to post-translational modification by phosphorylation on tyrosine and serine residues. Recent evidence suggested that STATs are methylated on a conserved arginine residue within the N-terminal region. STAT arginine methylation has been described to be important for STAT function and loss of arginine methylation was discussed to be involved in interferon resistance of cancer cells. Here we provide several independent lines of evidence indicating that the issue of arginine methylation of STATs has to be reassessed. First, we show that treatment of melanoma and fibrosarcoma cells with inhibitors used to suppress methylation (N-methyl-2-deoxyadenosine, adenosine, DL-homocysteine) had profound and rapid effects on phosphorylation of STAT1 and STAT3 but also on p38 and Erk signaling cascades which are known to cross-talk with the Jak/STAT pathway. Second, we show that anti-methylarginine antibodies did not precipitate specifically STAT1 or STAT3. Third, we show that mutation of Arg³¹ to Lys led to destabilization of STAT1 and STAT3, implicating an important structural role of Arg³¹. Finally, purified catalytically active protein arginine methyltransferases (PRMT1, -2, -3, -4, and -6) did not methylate STAT proteins, and co-transfection with PRMT1 did not affect STAT1-controlled reporter gene activity. Taken together, our data suggest the absence of arginine methylation of STAT1 and STAT3.