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DNA topoisomerase II (topo II) is an essential nuclear enzyme which catalyzes the interconversions of various forms of DNA. As predicted from the human topo II cDNA, the enzyme contains a potential leucine zipper protein dimerization motif. We therefore tested whether topo II could enter protein-protein interactions with other better characterized leucine zipper-containing proteins and determined if these interactions could modify topo II enzymatic activity in vitro. By far Western analyses, a large C-terminal fragment of human topo II was shown to interact with the DNA binding and dimerization regions of either cAMP response element binding protein (CREB) or the activating transcription factor-2. The C-terminal topo II fragment also interacted with full-length c-Jun, but not with full-length c-Fos. Using CREB as a prototype, the effect of this interaction on various topo II catalytic activities was assessed in vitro. CREB, at a 1- to 10-fold molar excess relative to topo II, inhibited site-specific DNA cleavage activity on a 242-base pair fragment of the human alpha-glycoprotein hormone subunit gene promoter. Very high CREB concentrations (400-fold excess) apparently inhibited topo II DNA relaxation activity, but this result was likely a direct effect of CREB on the topology of the DNA substrate. More interestingly, a 10-fold molar excess of CREB stimulated topo II decatenation activity, the essential function of this enzyme in cell division. This stimulatory effect could also be elicited by c-Jun, which interacts with topo II, but not by c-Fos, which does not bind topo II in our in vitro assay. Since similar amounts of CREB reduced the abundance of topo II DNA cleavage products from the human alpha-CG promoter yet also stimulated decatenation activity, it can be concluded that either: 1) CREB stimulated the religation rate of topo II; or 2) CREB directed topo II to a new cleavage site present on the decatenation substrate but not present on the limited alpha-CG promoter. The structural requirements for topo II protein-protein interactions were also investigated. Site-directed mutations which destroyed the putative topo II leucine zipper did not disrupt topo II protein-protein interactions. Since the putative leucine zipper in topo II does not appear to mediate protein-protein interactions, we propose that an alternate as yet uncharacterized structure is involved in the association of topo II with itself and other regulatory proteins.

molecular endocrinology

Modification of DNA topoisomerase II activity via direct interactions with the cyclic adenosine-3',5'-monophosphate response element-binding protein and related transcription factors.