Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats
Fig 5
High-resolution structures of MORN repeat proteins and a structural redefinition of the MORN repeat.
(A) Schematic depiction and crystal structure of the TbMORN1(7–15) dimer in its P21 crystal form. Amino acid numbers and N- and C-termini are indicated in the schematic. The crystal structure is shown in two orientations, with main dimensions indicated in Å. The structure contains 2 x 9 MORN repeats and is an antiparallel homodimer with the subunits arranged in a splayed tail-to-tail configuration. The secondary structure consists of exclusively antiparallel beta-strands and peripherally positioned loops, which together form a longitudinal groove through the middle of the protein. (B) Crystal structure of TgMORN1(7–15) shown in two orientations. The number of MORN repeats and the configuration is the same as for TbMORN1(7–15) in panel A. (C) Crystal structure of PfMORN1(7–15) shown in two orientations. The bound Zn2+ ion is labelled. The structure contains 2 x 9 MORN repeats, again in tail-to-tail configuration but with an overall V-shaped arrangement. (D) Alignment of all 9 TbMORN1(7–15) MORN repeats in the crystal structure reveals a high level of structural conservation. (E) A revised consensus MORN repeat sequence, based on the crystal structures. A new alignment of the MORN repeats in TbMORN1 is shown. Repeats 7–15 are present in the crystal structure; repeats 1–6 are inferred. Conservation of sequence identity is indicated by colour intensity. Each MORN repeat consists of a β-hairpin, built up of two 6-residue β-strands connected by a 5-residue loop. The β-hairpin is followed by a 6-residue loop that connects to the next MORN repeat. The new 23-residues long consensus MORN repeat starts with the GxG motif. (F) The tertiary structure of individual MORN repeats is stabilised by hydrogen bonds between the first G of the GxG motif and the W from the YEGEW motif. MORN repeat arrays are further stabilised by aromatic stacking between the highly conserved aromatic residues in the YEGEW and LxY motifs, and by T-shaped π-stacking interactions of the highly conserved Y of the YEGEW motif, which is sandwiched between the W residue of its own motif, and the W residue in the next YEGEW motif. (G) A single TbMORN1(7–15) subunit viewed at an oblique angle. The residues of the YEGEW and LxY motifs involved in aromatic stacking line the surface of the longitudinal groove running through the middle of the protein.