Identification of an mRNA-binding protein and the specific elements that may mediate the pH-responsive induction of renal glutaminase mRNA.

Various segments of the 3'-nontranslated region of the renal glutaminase (GA) mRNA were tested for their ability to enhance turnover and pH responsiveness. The combined effects were retained in the 340-base R-2 segment. However, the combined R-1 and R-3 fragments also imparted a partial destabilization and pH responsiveness to a chimeric beta-globin mRNA. RNA electrophoretic mobility shift assays indicated that cytosolic extracts of rat renal cortex contain a protein that binds to the R-2 and R-3 RNAs. The binding observed with the R-2 RNA was mapped to a direct repeat of an 8-base AU sequence. This binding was effectively competed with an excess of the same RNA, but not by adjacent or unrelated RNAs. UV cross-linking experiments identified a 48-kDa protein that binds to the AU repeats of the R-2 RNA. The apparent binding of this protein was greatly reduced in renal cytosolic extracts prepared from acutely acidotic rats. Two related RNA sequences in the R-3 segment also exhibited specific binding. However, the latter binding was more effectively competed by R-2 RNA than by itself, indicating that the homologous sites may be weaker binding sites for the same 48-kDa protein. Thus, a single protein may bind specifically to multiple instability elements within the 3'-nontranslated region of the GA mRNA and mediate its pH-responsive stabilization.