PT - JOURNAL ARTICLE AU - Sara Sajko AU - Irina Grishkovskaya AU - Julius Kostan AU - Melissa Graewert AU - Kim Setiawan AU - Linda Trübestein AU - Korbinian Niedermüller AU - Charlotte Gehin AU - Antonio Sponga AU - Martin Puchinger AU - Anne-Claude Gavin AU - Thomas Leonard AU - Dimitri Svergun AU - Terry K. Smith AU - Brooke Morriswood AU - Kristina Djinovic-Carugo TI - Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats AID - 10.1101/826180 DP - 2020 Jan 01 TA - bioRxiv PG - 826180 4099 - http://biorxiv.org/content/early/2020/01/31/826180.short 4100 - http://biorxiv.org/content/early/2020/01/31/826180.full AB - 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 repeat motifs such as leucine-rich repeats, armadillo repeats, WD40 repeats, and ankyrin repeats. In functional terms, they are often assumed to be lipid-binding modules that mediate membrane targeting, but direct evidence for this role is actually lacking. This putative activity was addressed by focusing on a protein composed solely of MORN repeats - Trypanosoma brucei MORN1. No evidence for binding to membranes or lipid vesicles by TbMORN1 either in vivo or in vitro could be obtained. TbMORN1 did interact with individual phospholipids but the potential physiological role of this interaction in trypanosomes remains unclear. 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. The structures indicated that MORN repeats can mediate homotypic interactions, and can function as both dimerisation and oligomerisation devices.