Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats
Fig 6
Dimerisation interfaces of TbMORN1(7–15) and PfMORN1(7–15).
(A) The dimerisation interface of TbMORN1(7–15) P21 involves residues from MORN repeats 12–15, which stabilise the dimer interface via aromatic π-stacking (Tyr330 and Phe345 from the respective subunits), hydrophobic interactions (Leu301, Leu347, Ile339, Ile322, Leu324), and additionally via hydrogen bonding interactions at the edges of the dimer interface. In comparison to the TbMORN1(7–15) C2 crystal structure, there are no disulphide bridges stabilising the dimerisation interface of TbMORN1(7–15) P21. (B) The dimerisation interface of the V-shaped PfMORN1(7–15) dimer is smaller and is additionally stabilised by the incorporation of a structural Zn2+ ion, which is tetrahedrally coordinated by Cys306 and Asp309 residues from each respective subunit. Thr311 holds the side chain of Asp309 in the appropriate orientation. The dimer interface is additionally stabilised by symmetric hydrogen bonding between the Thr311 pair, aromatic stacking between the Phe330 pair, a hydrophobic cluster formed by Leu328, Val331 and Leu336, two salt bridges between Lys322 and Glu308 from the respective subunits, anion-π interaction of a side chain of Glu308 with Phe304, and a combination of aromatic stacking (His334 pair) and polar interactions (His334, Asn332) at the vertex of the dimer. (C) An electrostatic map calculated for a single sub-unit of TbMORN1(7–15). Vacuum electrostatics were calculated in PyMOL, where red denotes -65.7 kT and blue denotes +65.7 kT. The structure on the right shows the predicted effect of two point mutations, R293A from MORN repeat 13, and K316A from MORN repeat 14. The mutations are expected to result in the loss of a positively-charged patch close to the dimer interface, and consequently disrupt the dimerisation of the TbMORN1(2–15) constructs.