IL-2 receptor (Tac antigen) protein expression is down-regulated by the 5'-untranslated region of the mRNA.
We have investigated the regulatory role of the 5'-untranslated region (5'-UTR) of the IL-2R mRNA. We noticed a region of striking homology (92%) between the human and bovine IL-2R cDNA in a stretch of 26 nucleotides located in the 5'-UTR. Within this 26 nucleotide region is an AUG that is out of frame with the IL-2R coding sequence. The murine IL-2R cDNA has an 11 bp direct repeat in the 5'-UTR that includes an upstream AUG, and this sequence is identical to the translational start site for the murine IL-2R protein. These observations raised the possibility that the two upstream AUG start codons might down regulate translation of the IL-2R by acting as false translational start sites. To investigate the possibility of IL-2R translational control, we examined sucrose gradient polysome profiles from Con A stimulated murine CD4+ splenocytes. The IL-2R mRNA was found in the portion of the gradient where free RNA, mono, and disomes migrate, whereas actively translated mRNA (lymphokines and beta-actin) were found in the portion of the gradient that contained the polysome fractions. This finding was consistent with translational down-regulation of the IL-2R mediated by ribosomal binding to the 5'-UTR start sites. We next examined cells transfected with the IL-2R cDNA that had the 5'-UTR deleted and compared protein expression to cells transfected with the full length construct. Flow microfluorometry analysis of cell surface IL-2R expression, showed that a clone transfected with the 5'-UTR deletion expressed five- to six-fold more IL-2R than a clone transfected with the full-length construct, even though both clones produced equivalent IL-2R mRNA levels. In clones transfected with the full length construct, the IL-2R mRNA associated with a reduced number of ribosomes compared to the mRNA in the deleted construct clones. These results indicate that sequences in the 5'-UTR of IL-2R mRNA lead to a decrease in the amount of ribosomes bound per IL-2R RNA molecule, and suggest that the level of IL-2R expression can thus be translationally down-regulated. This is the first growth factor receptor shown to be posttranscriptionally controlled at the translational level, and these findings have important implications for IL-2R synthesis and cell surface expression of this immunologically active cell surface receptor.
Weinberg
A D
AD
Department of Biology, University of California, San Diego, La Jolla 92093.
Swain
S L
SL
D000818:Animals; D001483:Base Sequence; D002417:Cattle; D005786:Gene Expression Regulation; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D011132:Polyribosomes / Q000378:metabolism; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics*; D015375:Receptors, Interleukin-2 / Q000235:genetics*; D012045:Regulatory Sequences, Nucleic Acid; D012270:Ribosomes / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid; D014162:Transfection
D012333:RNA, Messenger; D015375:Receptors, Interleukin-2