ENHANCED STABILITY OF INTERLEUKIN-2 MESSENGER-RNA IN MLA-144 CELLS - POSSIBLE ROLE OF CYTOPLASMIC AU-RICH SEQUENCE-BINDING PROTEINS

The MLA 144 gibbon T cell line is infected with a type C retrovirus an d constitutively expresses interleukin-2 (IL-2) and granulocyte macrop hage colony-stimulating factor (GM-CSF). IL-2 mRNA levels are 10-fold more abundant than GM-CSF in these cells. Comparable transcriptional r ates for these lymphokines suggested the involvement of post-transcrip tional mechanisms in selective IL-2 mRNA accumulation, IL-2 mRNA is ex ceptionally stable in MLA cells with a t(1/2) of more than 8 h. The pr esence of reiterated AUUUA sequences in the 3'-untranslated region (UT R) has been shown to confer mRNA lability. The provirally altered MLA IL-2 allele encodes an mRNA in which three AUUUA motifs have been dele ted. Six major cytoplasmic proteins bound in vitro transcribed RNA pro bes containing sequences from the 3'-UTR of normal human IL-2 (3'-IL-2 ), GM-CSF (Delta 2R1), and the virally altered MLA IL-2 (3'-IL-2 PV) m RNA. Increased binding of these proteins to 3'-IL-2 PV was observed re lative to 3'-IL-2 or Delta 2R1. Northwestern blotting demonstrated sim ilar differential ability of a 36- and 43-kDa protein to bind, as well as showed that these proteins colocalized by immunoblotting as hnRNP Al and C, respectively. These findings suggest a direct correlation be tween differential binding of cytoplasmic proteins to AU-rich 3'-UTRs in vitro and lymphokine mRNA stability in vivo.