Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats

Abstract

MORN (Membrane Occupation and Recognition Nexus) repeat proteins have a wide taxonomic distribution, being found in both prokaryotes and eukaryotes. Despite this ubiquity, they remain poorly characterised at both a structural and a functional level compared to other common repeats. In functional terms, they are often assumed to be lipid-binding modules that mediate membrane targeting. We addressed this putative activity by focusing on a protein composed solely of MORN repeats—Trypanosoma brucei MORN1. Surprisingly, no evidence for binding to membranes or lipid vesicles by TbMORN1 could be obtained either in vivo or in vitro. Conversely, TbMORN1 did interact with individual phospholipids. High- and low-resolution structures of the MORN1 protein from Trypanosoma brucei and homologous proteins from the parasites Toxoplasma gondii and Plasmodium falciparum were obtained using a combination of macromolecular crystallography, small-angle X-ray scattering, and electron microscopy. This enabled a first structure-based definition of the MORN repeat itself. Furthermore, all three structures dimerised via their C-termini in an antiparallel configuration. The dimers could form extended or V-shaped quaternary structures depending on the presence of specific interface residues. This work provides a new perspective on MORN repeats, showing that they are protein-protein interaction modules capable of mediating both dimerisation and oligomerisation.

The role of junctophilin proteins in cellular function

“…A recent in vitro lipid-binding study of over 100 proteins of the protozoan ciliate Tetrahymena thermophila confirmed the lipid-binding capacity for similar 14-aa long MORN consensus motifs ( 80 ). In contrast, it appears that the negative surface charges on the parasite MORN domains exclude a phospholipid-binding function, both rather mediate binding to specific cytoskeletal protein domains ( 81 ). However, both parasitic and mammalian MORN domain structures are capable of binding specific cytoskeletal protein domains as discussed in sect.…”

“…However, both parasitic and mammalian MORN domain structures are capable of binding specific cytoskeletal protein domains as discussed in sect. 2.2.3 ( 81 ).…”

The Builders of the Junction: Roles of Junctophilin1 and Junctophilin2 in the Assembly of the Sarcoplasmic Reticulum–Plasma Membrane Junctions in Striated Muscle

“…This particular organization allows the junctophilins to link the ER membrane and the plasma membrane to one another. The MORN motifs are known to bind to the internal leaflet of the plasma membrane and specifically to phosphatidylinositol phosphate species [ 29 , 30 , 31 ], although the role of the MORN motif as a bona fide membrane-binding domain has been recently called into question after the observation that a number of proteins that also contain MORN repeats are either not associated with membranes or employ alternative domains to anchor to the membrane, or utilize their MORN motifs as protein-protein–protein interaction domains rather than membrane association domains [ 32 , 33 , 34 ]. Therefore, the ability of the MORN motifs in the junctophilins to associate with the membrane might arise from the presence of positively charged residues in the less conserved regions or from post-transcriptional modifications.…”

The Role of Lysine Methyltransferase SET7/9 in Proliferation and Cell Stress Response

“…It is generally assumed that MORN repeats bind to lipids and are responsible for plasma membrane targeting. However, there is a growing volume of evidence suggesting that MORN repeats may mediate protein-protein interactions [ 24 , 25 ]. Indeed, by using a GST-pull-down assay coupled with mass spectrometry, we have demonstrated that MORN repeats are responsible for the majority of SET7/9 interactions [ 10 ].…”