title: Molecular mechanisms underlying he interaction of motuporin and microcystins with type-1 and type-2A protein phosphatases.
abstract: Heptapeptide microcystin and pentapeptide motuporin (nodularin-V) are equipotent inhibitors of type-1 and type-2A protein phosphatase catalytic subunits (PP-1c and PP-2Ac). Herein we describe elucidation of the molecular mechanisms involved in the interaction of these structurally similar hepatotoxins with PP-1c/PP-2Ac and identification of an important functional difference between their mode of interaction with these enzymes. Microcystin-LR, microcystin-LA, and microcystin-LL were found to interact with PP-2Ac and PP-1c by a two-step mechanism involving rapid binding and inactivation of the protein phosphatase (PPase) catalytic subunit, followed by a slower covalent interaction (within hours). Covalent adducts comprising PPase-toxin complexes were separated from free PPase by C-18 reverse-phase liquid chromatography, thus allowing the time course of covalent adduct formation to be quantitated. In contrast to microcystins, motuporin (nodularin-V) and nodularin-R were unable to form covalent complexes with either PP-1c or PP-2Ac even after 96 h incubation. Specific reduction of microcystin-LA to dihydromicrocystin-LA abolished the ability of the toxin to form a covalent adduct with PP-2Ac. Specific methyl esterification of the single Glu residue in microcystin-LR rendered this toxin inactive as a PPase inhibitor and abolished subsequent formation of a covalent adduct. Our data indicate that inactivation of PP-2Ac/PP-1c by microcystins precedes covalent modification of the PPases via a Michael addition reaction between a nucleophilic phosphatase residue and Mdha in the heptapeptide toxin. In contrast, following rapid inactivation of PP-2Ac/PP-1c by motuporin, the equivalent N-methyldehydrobutyrine residue in this toxin is unreactive and does not form a covalent bond with the PPases. These results are consistent with structural data for (i) the NMR solution structures of microcystin-LR and motuporin, which indicate a striking difference in the relative positions of their corresponding dehydroamino acids in the toxin peptide backbone, and (ii) X-ray crystallographic data on an inactive complex between PP-1c and microcystin-LR, which show a covalent bond between Cys-273 and the bound toxin.
authors.affiliation: Department of Biochemistry, University of Alberta, Edmonton, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Craig
authors.forename: H A
authors.initials: HA
authors.lastname: Luu
authors.forename: T L
authors.initials: TL
authors.lastname: McCready
authors.forename: D
authors.initials: D
authors.lastname: Williams
authors.forename: R J
authors.initials: RJ
authors.lastname: Andersen
authors.forename: C F
authors.initials: CF
authors.lastname: Holmes
mesh_terms: D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic; C093605:motuporin; C063998:nodularin; C093607:microcystin LL; C049865:cyanoginosin-LA; D010749:Phosphoprotein Phosphatases; C057862:cyanoginosin LR