Browsing by Subject "Phenol coupling"

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    Heterologe Expression und molekulare Charakterisierung des Dirigentproteins AtDIR6 aus A. thaliana
    (2011) Pickel, Benjamin; Schaller, Andreas
    Dirigent proteins are involved in the stereo- and regioselective control of plant secondary metabolism. Functionally described dirigent proteins from Forsythia intermedia and Thuja plicata couple coniferyl alcohol radicals to (+)-pinoresinol, a precursor of various lignans including the pharmaceutically relevant podophyllotoxin. The discovery of (?)-lariciresinol in A. thaliana roots and the accumulation of its precursor (?)-pinoresinol in a knock-out mutant lacking two pinoresinol reductases indicated the presence of a novel dirigent activity in A. thaliana which mediates the enantiocomplementary formation of (?)-pinoresinol. In this work AtDIR6 was identified as a candidate for this novel dirigent activity. The protein was cloned and heterologously expressed in a plant cell culture system. The recombinant protein was purified to appearent homogeneity by conventional chromatography methods. The purified protein was functionally active and directed the coupling of coniferyl alcohol radicals to (?)-pinoresinol in vitro. It was further shown that the stereoselectivity of AtDIR6 is opposed to that of known dirigent proteins in F. intermedia and T. plicata, and therefore, AtDIR6 is the first of the long-sought nantiocomplementary dirigent proteins. AtDIR6 was shown to possess a N-terminal signal peptide, which was cleaved during secretion between amino acids 29 and 30. Mature AtDIR6 accumulated extracellularly and remained non-covalently attached to the primary cell wall of suspension cultured cells. The native protein is glycosylated with two complex type and paucimannosidic Nglycans, respectively. It forms homodimers of app. 42 kDa and shows a high content of b-sheets. The functionally described dirigent proteins are small proteins that are characterised by the ability to bind coniferyl alcohol radicals and couple them enantiospecifically without possessing a catalytic activity of their own. Sequence identity between different dirigent proteins may be low. In these aspects dirigent proteins are similiar to lipocalins. A sequence alignment with dirigent proteins and lipocalins shows that the lipocalin-specific sequence motive, which is part of SCR II, is conserved among all functionally described dirigent proteins. Structural and mechanistic features of AtDIR6 suggest that dirigent proteins may belong to the calycin superfamily, which also includes lipocalins, and that their threedimensional structure may be that of a b-barrel.