Poly(A) binding protein (PABP) binds mRNA poly(A) tails and affects mRNA 
stability and translation. We show here that there is little free PABP in NIH3T3 
cells, with the vast majority complexed with RNA. We found that PABP in NIH3T3 
cytoplasmic lysates and recombinant human PABP can bind to AU-rich RNA with high 
affinity. Human PABP bound an AU-rich RNA with Kd in the nm range, which was 
only sixfold weaker than the affinity for oligo(A) RNA. Truncated PABP 
containing RNA recognition motif domains 3 and 4 retained binding to both 
AU-rich and oligo(A) RNA, whereas a truncated PABP containing RNA recognition 
motif domains 1 and 2 was highly selective for oligo(A) RNA. The inducible PABP, 
iPABP, was found to be even less discriminating than PABP in RNA binding, with 
affinities for AU-rich and oligo(A) RNAs differing by only twofold. These data 
suggest that iPABP and PABP may in some situations interact with other RNA 
regions in addition to the poly(A) tail.