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 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 it remains unclear if this was physiological or an artefact. 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.
