InVitroBinding ofHumanT-Cell LeukemiaVirus rex Proteins to therex-Response Element ofViral Transcripts

HumanT-celi leukemia virus(HTLV-I, HTLV-II) rex protein function isrequired forthecytoplasmic expression ofincompletely spliced viral transcripts encoding structural proteins. Theeffect ismediated byacisacting rex-responseelement (RRX)whichislocated near the3'endofallviral mRNAs.We showthatrex polypeptides ofHTLV-IandHTLV-IIexpressed inEscherichia coli arecapable ofspecifically binding RRXcontaining transcripts ofbothviruses incell-free assays.Binding analyses withdeletion variants ofrexproteins revealed adomainwithRNA-binding activity inthefirst 77N-terminal aminoacids. Removalofa basic peptide of19aminoacids fromtheN terminus abrogated RNA binding, whereas a l-galactosidase fusion protein containing this peptide boundtotheRRX.Theseresults suggest thatdirect binding ofrexprotein totheRRX isimportant forrex-mediated regulation ofviral geneexpression andthat a short stretch ofpositively charged aminoacids contributes tothespecific binding ofrextoits target RNA.