Structural Biochemistry Laboratory, Department of Chemistry & Biochemistry, Mail Code 1030, 5500 Campanile Drive, San Diego State University, San Diego, CA 92182-1030, USA.
NF-kappaB activation in response to pro-inflammatory stimuli relies upon phosphorylation of IkappaB alpha at serines 32 and 36 by the beta subunit of the IkappaB kinase complex (IKK). In this study, we build upon the observation that highly purified human IKKbeta subunit preparations retain this specificity in vitro. We show that IKKbeta constructs that lack their carboxy-terminus beginning at the leucine zipper motif fail to phosphorylate IkappaB alpha at Ser-32 and Ser-36. Rather, these constructs, which contain the entire IKKbeta subunit kinase domain, phosphorylate serine and threonine residues contained within the IkappaB alpha carboxy-terminal PEST region. Furthermore, removal of the leucine zipper and helix-loop-helix regions converts IKKbeta to monomer. We propose that the helix-loop-helix of the human IKKbeta subunit is necessary for restricting substrate specificity toward Ser-32 and Ser-36 in IkappaB alpha and that in the absence of its carboxy-terminal protein structural motifs the human IKKbeta subunit kinase domain exhibits a CK2-like phosphorylation specificity.