The liverwort Marchantia polymorpha expresses orthologs of the fungal Agaricus bisporus agglutinin family

Abstract

A lectin different from the previously described mannose-binding agglutinins has been isolated from the liverwort Marchantia polymorpha. Biochemical characterization of the purified lectin combined with the data from earlier transcriptome analyses demonstrated that the novel M. polymorpha agglutinin is not related to any of the known plant lectin families, but closely resembles the Agaricus bisporus-type lectins, which hitherto have been found exclusively in fungi. Immunolocalization studies confirmed that lectin is exclusively associated with plant cells, ruling out the possibility of a fungal origin. Extensive screening of publicly accessible databases confirmed that, apart from fungi, the occurrence of A. bisporus-type lectins is confined to M. polymorpha and the moss Tortula ruralis. Expression of a typical fungal protein in a liverwort and a moss raises the question of the origin of the corresponding genes. Regardless of the evolutionary origin, the presence of a functional A. bisporus lectin ortholog in M. polymorpha provides evidence for the expression of an additional carbohydrate-binding domain in Viridiplantae.

Figure 1.

Structural alignment of the amino acid sequences of M. polymorpha ABA orthologs (MarpoABA), the Tortula ABA ortholog (TorruABA), and ABA. Strands of β-sheet (β1–β10) and stretches of α-helix (α1, α2) occurring along the polypeptide chain of ABA have been indicated.

Figure 2.

Localization of MarpoABA in thalli and callus of M. polymorpha. A to C, Transverse sections of callus cells (A) and of upper (B) and lower (C) cell layers of the liverwort thallus after staining with toluidine blue. Note the rich content of parenchyma cells below the epidermal layer. Arrows in C indicate transverse sections of rhizoids. D to F, Confocal images of immunolabeled sections from the corresponding callus (D) and thallus (E and F) cells showing immunolabeling essentially restricted to the cell contours. Scale bars = 20 μm.

Figure 3.

Molecular modeling of ABA orthologs from Marchantia and Tortula. A, Ribbon diagram of the ABA protomer showing the β-sandwich organization made of two bundles of β-sheets ([β1, β3, β4, β9, β10] and [β2, β5, β6, β7, β8]) interconnected by a helix-loop-helix consisting of two α-helices, α1 and α2. N and C correspond to the N- and C-terminal ends of the polypeptide chain, respectively. The location of the T-antigen-binding site (white star) and GlcNAc-binding site (black star) are indicated by stars. B, Ribbon diagram of the modeled MarpoABA2a. C, Ribbon diagram of the modeled TorruABA. D, Network of hydrogen bonds (dark dashed lines) connecting the T-antigen disaccharide (Galβ1,3GalNAc; black sticks) to the amino acid residues forming the carbohydrate-binding site of ABA. E, Docking of the T-antigen disaccharide (Galβ1,3GalNAc; black sticks) in the carbohydrate-binding site of the modeled MarpoABA2a. F, Network of hydrogen bonds (dark dashed lines) connecting GlcNAc (black sticks) to the amino acid residues forming the carbohydrate-binding site of ABA. G, Docking of GlcNAc (black sticks) in the carbohydrate-binding site of the modeled MarpoABA1a.

Figure 4.

Phylogenetic tree of ABA orthologs from fungi and plants. MarpoABA and TorruABA refer to the expressed plant proteins from M. polymorpha and T. ruralis, respectively. Basidiomycota lectins are Agabi (ABA; AAA85813); paxin (Paxillus involutus expressed proteins; AAT91249); PlecoI (Pleurotus cornucopiae lectin PCL-F1; BAB63922); PlecoII (Pleurotus cornucopiae lectin PCL-F2; BAB63923); Phach1 (Phanerochaete chrysosporium hypothetical protein encoded by complement AADS01000313.1:6707–7153); and Xerch (Xerocomus chrysenteron lectin XCL1; AL73235). Ascomycota lectins are Artol (Arthrobotrys oligospora lectin; CAA65781); Gibze (Gibberella zeae hypothetical expressed protein EAA76455); Neucr (Neurospora crassa hypothetical expressed protein EAA30976); Podan (Podospora anserina hypothetical protein CAD60779); and Sorma (Sordaria macrospora hypothetical protein CAH03681). Figures refer to GenBank accession numbers. The scale bar indicates the number of amino acid changes.