Unraveling the Activation Mechanism of Taspase1 which Controls the Oncogenic AF4–MLL Fusion Protein

• Published in: EBioMedicine 2015-05, Vol.2 (5), p.386-395
• Main Authors: Sabiani, Samaneh, Geppert, Tim, Engelbrecht, Christian, Kowarz, Eric, Schneider, Gisbert, Marschalek, Rolf
• Format: Article
• Language: English
• Subjects: AF4–MLL; Crystallography, X-Ray; DNA Mutational Analysis; Endopeptidases - chemistry; Endopeptidases - metabolism; Enzyme Activation; Enzyme Assays; Fluorescence Resonance Energy Transfer; Genes, Reporter; HEK293 Cells; Humans; Hydrolysis; Models, Molecular; Mutagenesis, Site-Directed; Mutant Proteins - metabolism; Myeloid-Lymphoid Leukemia Protein - metabolism; Oncogene Proteins, Fusion - metabolism; Oncoprotein activation; Original; Protein Multimerization; Reproducibility of Results; t(4; 11) leukemia; Taspase1
• ISSN: 2352-3964; 2352-3964
• DOI: 10.1016/j.ebiom.2015.04.009

We have recently demonstrated that Taspase1-mediated cleavage of the AF4–MLL oncoprotein results in the formation of a stable multiprotein complex which forms the key event for the onset of acute proB leukemia in mice. Therefore, Taspase1 represents a conditional oncoprotein in the context of t(4;11) leukemia. In this report, we used site-directed mutagenesis to unravel the molecular events by which Taspase1 becomes sequentially activated. Monomeric pro-enzymes form dimers which are autocatalytically processed into the enzymatically active form of Taspase1 (αββα). The active enzyme cleaves only very few target proteins, e.g., MLL, MLL4 and TFIIA at their corresponding consensus cleavage sites (CSTasp1) as well as AF4–MLL in the case of leukemogenic translocation. This knowledge was translated into the design of a dominant-negative mutant of Taspase1 (dnTASP1). As expected, simultaneous expression of the leukemogenic AF4–MLL and dnTASP1 causes the disappearance of the leukemogenic oncoprotein, because the uncleaved AF4–MLL protein (328kDa) is subject to proteasomal degradation, while the cleaved AF4–MLL forms a stable oncogenic multi-protein complex with a very long half-life. Moreover, coexpression of dnTASP1 with a BFP-CSTasp1-GFP FRET biosensor effectively inhibits cleavage. The impact of our findings on future drug development and potential treatment options for t(4;11) leukemia will be discussed. •Taspase1 has coevolved with the Trithorax/MLL protein family.•Taspase1 hydrolyzes MLL and few other substrate proteins at consensus cleavage sites.•Taspase1 is a conditional oncoprotein in of solid and hematological cancers.•Taspase1 is required for the processing of the leukemogenic AF4–MLL fusion protein.•Inhibition of Taspase1 might have a great therapeutic potential.