The RNA-binding proteins AUF-1 and YB-1 participate in the β-complex. (A) The β-globin 3′UTR interacts with 2 trans-acting RBPs. K562 cytoplasmic extracts were incubated with agarose-linked ssDNAs encompassing the 15-nt WT₁₅ sequence or with control ssDNAs containing derivative sequences that do not (M3) or do (M10) support β-complex assembly. Retained proteins were resolved by SDS-PAGE. Open and closed arrows indicate differentially bound 42- and 47-kDa bands, respectively. (B) AUF-1 and YB-1 bind specifically to β-globin 3′UTR DNAs. Immunoblots were conducted on retentate from WT₁₅, M3₁₅, and M10₁₅ ssDNAs using YB-1 and AUF-1 Abs. Input is 5%. (C) AUF-1 and YB-1 are highly expressed in erythroid-induced CD34⁺ cells. Cytoplasmic extracts prepared from K562, uninduced (U), and erythroid-induced (I) human CD34⁺ cells were analyzed by immunoblot using factor-specific or control β-actin Abs.