Viperin is an interferon-inducible protein inhibiting many DNA and RNA viruses. It contains an N-terminal transmembrame helix, a highly conserved C-terminus and a middle region carrying a CX3CX2C motif, characteristic of radical 5-adenosyl-L-methionine (SAM) enzymes. So far no structural characterization has been reported and reconstitution of the [4Fe-4S] cluster in viperin all failed. Here, by dissecting the 361-residue human viperin into 12 fragments, followed by extensive CD and NMR characterization, Viperin (45-361) was identified to be soluble and structured in buffers. Most importantly, we have successfully reconstituted the [4Fe-4S] cluster in Viperin (45-361), thus providing the first experimental evidence confirming that viperin is indeed a radical SAM enzyme. Furthermore, the C-terminus Viperin (214-361) which is insoluble in buffers but again can be solubdized in salt-free water appears to be only partially folded. Our results thus imply that the radical SAM enzyme activity may play a key role in the broad antiviral actions of viperin. (C) 2009 Elsevier Inc. All rights reserved.