Control of Na+-K+-ATPase beta 1-subunit expression: role of 3'-untranslated region

Using in vitro translation and cell transfection assays, we previously demonstrated that the Na+ -K+ -ATPase beta1 mRNA species containing its longest 3'-untranslated region (UTR) exhibited the lowest translational efficiency. Here, employing deletions and in vivo expression assays, using direct injection of plasmids into rat ventricular myocardium, we identified a 143-nt segment located in the distal 3'-UTR of beta1 mRNA that was associated with decreased luciferase expression; interestingly, this segment contains three AUUUA motifs. Using RNA-protein binding assays and UV cross-linking of cRNA with cytosolic proteins of rat heart, we identified an approximately 38-kDa protein that specifically bound to the cRNA encoding the 143-nt segment of beta1 mRNA 3'-UTR. Mutation of three nucleotides located in the middle region of the 143-nt segment, which was predicted to greatly disrupt a putative stem-loop structure of the cRNA in this region, was associated with reduced binding of the mutated cRNA to the protein migrating at approximately 38 kDa. The cRNA encoding a segment of cyclooxygenase-2 mRNA 3'-UTR containing six AUUUA sequences did not bind the protein migrating at approximately 38 kDa and did not compete with the binding of the wild-type 143-nt beta(1) cRNA to the protein. The above results suggest that the 143-nt segment in the distal segment of the 3'-UTR of beta1 mRNA may play an important role in the control of beta1-subunit expression.