Protein Interactions of the MLL PHD Fingers Modulate MLL Target Gene Regulation in Human Cells

Abstract

The PHD fingers of the human MLL and Drosophila trx proteins have strong amino acid sequence conservation but their function is unknown. We have determined that these fingers mediate homodimerization and binding of MLL to Cyp33, a nuclear cyclophilin. These two proteins interact in vitro and in vivo in mammalian cells and colocalize at specific nuclear subdomains. Overexpression of the Cyp33 protein in leukemia cells results in altered expression ofHOX genes that are targets for regulation by MLL. These alterations are suppressed by cyclosporine and are not observed in cell lines that express a mutant MLL protein without PHD fingers. These results suggest that binding of Cyp33 to MLL modulates its effects on the expression of target genes.

ACKNOWLEDGMENTS

We are grateful to Richard Baer for providing the vector constructs for the mammalian two-hybrid system and to Yih-Sheng Yang for yeast two-hybrid control plasmids GBT-NonO and GAD-NonO. We thank Peter Harte and Stephanie L. Nelson for trithorax yeast two-hybrid constructs. We also thank Nancy Zeleznik-Le for GST-MLL fusion protein constructs and discussion of results and K. FitzGerald and U. Osmers for discussion and help in editing the manuscript.

This work was supported by U.S. Public Health Service Grant ROI CA38725 (to M.O.D.) and by Deutsche Forschungsgemeinschaft (SFB 388/B3) and the Fonds der Chemischen Industrie (to M.T.).