Involvement of TBL/DUF231 proteins into cell wall biology

Abstract

Through map-based cloning we determined TRICHOME BIREFRINGENCE (TBR) to belong to a plant-specific, yet anonymous gene family with 46 members in Arabidopsis thaliana. These genes all encode the domain of unknown function 231 (DUF231). TBR and its homolog TRICHOME BIREFRINGENCE-LIKE3 (TBL3) are transcriptionally coordinated with CELLULOSE SYNTHASE (CESA) genes, and loss of TBR or TBL3 results in decreased levels of crystalline secondary wall cellulose in trichomes and stems, respectively. Loss of TBR or TBL3 further results in increased pectin methylesterase (PME) activity and reduced pectin esterification in etiolated Arabidopsis hypocotyls. Together, the results suggest that DUF231 proteins might function in the maintenance of pectin- and probably homogalacturonan esterification, and that this is a requirement for normal secondary wall cellulose synthesis, at least in some tissues and organs. Here we expand the discussion about the role of TBL/DUF231 proteins in cell wall biology based on sequence and structure analyses. Our analysis revealed structural similarities of TBR with a rhamnogalacturonan acetylesterase (RGAE) of Aspergillus aculeatus and the protein LUSTRIN A-LIKE (Oryza sativa). The implications of these findings in regard to TBL functions are discussed.

Figure 1

Modeled spatial structure of the TBR fragment. (A) Superposition of the backbones of the TBR fragment model (amino acids 325–604; red) with the RGAE structure (Aspergillus aculeatus; PDB: 1pp4, blue). Likely active site residues in TBR (Ser, His, Asp), which are conserved in the alignment with RGAE, are shown as space filling balls. (B) Superposition of the backbones of the TBR fragment model (blue) with the X-ray PME structure (Daucus carota; PDB: 1GQ8, red). The active site residues in PME (Gln¹¹³, Gln¹³⁵, Asp¹³⁶ not visible, Asp¹⁵⁷) are shown as space filling balls. The aligned residues from TBR are Glu⁶⁶, Trp⁶⁷ and Thr⁷⁵ (Rmsd = 7.7 Å, Z-Score = 1.6). (C) Superposition of the backbones of the TBR fragment model (blue) with the X-ray structure of PME-I (Arabidopsis thaliana; PDB: 1X8Z, red; Rmsd = 4.4 Å Z-Score = 3.5). Modeling was done with the program Jackal. The superposition was computed with CE. The figures were generated with RasMol.