PU.1 recruits the binding of a second B cell-restricted nuclear factor, NF-EM5, to a DNA site in the immunoglobulin κ 3′ enhancer. DNA binding by NF-EM5 requires a protein-protein interaction with PU.1 and specific DNA contacts. Dephosphorylated PU.1 bound to DNA but did not interact with NF-EM5. Analysis of serine-to-alanine mutations in PU.1 indicated that serine 148 (Ser148) is required for protein-protein interaction. PU.1 produced in bacteria did not interact with NF-EM5. Phosphorylation of bacterially produced PU.1 by purified casein kinase II modified it to a form that interacted with NF-EM5 and that recruited NF-EM5 to bind to DNA. Phosphopeptide analysis of bacterially produced PU.1 suggested that Ser 148 is phosphorylated by casein kinase II. This site is also phosphorylated in vivo Expression of wild-type PU.1 increased expression of a reporter construct containing the PU.1 and NF-EM5 binding sites nearly sixfold, whereas the Ser148 mutant form only weakly activated transcription. These results demonstrate that phosphorylation of PU.1 at Ser148 is necessary for interaction with NF-EM5 and suggest that this phosphorylation can regulate transcriptional activity.
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