title: Crystal structures and mutagenesis of PPP-family ser/thr protein phosphatases elucidate the selectivity of cantharidin and novel norcantharidin-based inhibitors of PP5C.
abstract: Cantharidin is a natural toxin and an active constituent in a traditional Chinese medicine used to treat tumors. Cantharidin acts as a semi-selective inhibitor of PPP-family ser/thr protein phosphatases. Despite sharing a common catalytic mechanism and marked structural similarity with PP1C, PP2AC and PP5C, human PP4C was found to be insensitive to the inhibitory activity of cantharidin. To explore the molecular basis for this selectivity, we synthesized and tested novel C5/C6-derivatives designed from quantum-based modeling of the interactions revealed in the co-crystal structures of PP5C in complex with cantharidin. Structure-activity relationship studies and analysis of high-resolution (1.25Å) PP5C-inhibitor co-crystal structures reveal close contacts between the inhibitor bridgehead oxygen and both a catalytic metal ion and a non-catalytic phenylalanine residue, the latter of which is substituted by tryptophan in PP4C. Quantum chemistry calculations predicted that steric clashes with the bulkier tryptophan side chain in PP4C would force all cantharidin-based inhibitors into an unfavorable binding mode, disrupting the strong coordination of active site metal ions observed in the PP5C co-crystal structures, thereby rendering PP4C insensitive to the inhibitors. This prediction was confirmed by inhibition studies employing native human PP4C. Mutation of PP5C (F446W) and PP1C (F257W), to mimic the PP4C active site, resulted in markedly suppressed sensitivity to cantharidin. These observations provide insight into the structural basis for the natural selectivity of cantharidin and provide an avenue for PP4C deselection. The novel crystal structures also provide insight into interactions that provide increased selectivity of the C5/C6 modifications for PP5C versus other PPP-family phosphatases.
authors.affiliation: Department of Medicine, University of Alabama Birmingham, Birmingham, AL 35294, USA.
authors.forename: Debasish
authors.initials: D
authors.lastname: Chattopadhyay
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Swingle
authors.affiliation: Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Edward A
authors.initials: EA
authors.lastname: Salter
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA; Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Wood
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Brandon
authors.initials: B
authors.lastname: D'Arcy
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA; Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Catherine
authors.initials: C
authors.lastname: Zivanov
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Kevin
authors.initials: K
authors.lastname: Abney
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Alla
authors.initials: A
authors.lastname: Musiyenko
authors.affiliation: Department of Biochemistry, Geisel School of Medicine at Dartmouth, Lebanon, NH 03756, USA.
authors.forename: Scott F
authors.initials: SF
authors.lastname: Rusin
authors.affiliation: Department of Biochemistry, Geisel School of Medicine at Dartmouth, Lebanon, NH 03756, USA; Norris Cotton Cancer Center, Geisel School of Medicine at Dartmouth, Lebanon, NH 03756, USA.
authors.forename: Arminja
authors.initials: A
authors.lastname: Kettenbach
authors.affiliation: Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Larry
authors.initials: L
authors.lastname: Yet
authors.affiliation: Department of Medicinal Chemistry, University of Kansas Specialized Chemistry Center, Lawrence, KS 66047, USA.
authors.forename: Chad E
authors.initials: CE
authors.lastname: Schroeder
authors.affiliation: Department of Medicinal Chemistry, University of Kansas Specialized Chemistry Center, Lawrence, KS 66047, USA.
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Golden
authors.affiliation: Centre for Systems Biology, Lunenfeld-Tanenbaum Research Institute at Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada; Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Wade H
authors.initials: WH
authors.lastname: Dunham
authors.affiliation: Centre for Systems Biology, Lunenfeld-Tanenbaum Research Institute at Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada; Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Anne-Claude
authors.initials: AC
authors.lastname: Gingras
authors.affiliation: Northeastern Collaborative Access Team (NE-CAT) Cornell University, Lemont, IL, USA.
authors.forename: Surajit
authors.initials: S
authors.lastname: Banerjee
authors.affiliation: Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: David
authors.initials: D
authors.lastname: Forbes
authors.affiliation: Department of Chemistry, University of South Alabama, Mobile, AL 36688, USA.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Wierzbicki
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, AL 36688, USA. Electronic address: rhonkanen@southalabama.edu.
authors.forename: Richard E
authors.initials: RE
authors.lastname: Honkanen
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry*; D002193:Cantharidin / Q000737:chemistry*; D020134:Catalytic Domain; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D004791:Enzyme Inhibitors / Q000737:chemistry*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D007700:Kinetics; D062105:Molecular Docking Simulation; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D000072417:Protein Domains; D054645:Protein Phosphatase 1 / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D013329:Structure-Activity Relationship
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D011994:Recombinant Proteins; C069741:norcantharidin; C517187:PPP1CA protein, human; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; C080406:protein phosphatase 4; C089641:protein phosphatase 5; C106079:protein phosphatase 6; D002193:Cantharidin
keywords: Crystal structures; Inhibitors; PP5C; Phosphatase; Screening