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

Sara Sajko,Linda Trübestein,Julius Kostan,Anne-Claude Gavin,Dimitri I Svergun,Kim Setiawan,Melissa Graewert,Thomas A Leonard,Kristina Djinovic-Carugo,Irina Grishkovskaya,Charlotte Gehin,Terry K Smith,Antonio Sponga,Korbinian Niedermüller,Martin Puchinger,Brooke Morriswood

doi:10.1371/journal.pone.0242677

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.

Highlights

MORN (Membrane Occupation and Recognition Nexus) repeats were first discovered in 2000, as a result of a screen for proteins present in the triad junctions of skeletal muscle [1]
TbMORN1 is composed of 15 consecutive 23-amino acid MORN repeats with a 5-amino acid extension after the sixth repeat (Fig 1A)
While there is abundant evidence that junctophilins associate with the plasma membrane and that the MORN repeat-containing region is likely to mediate this, there is to the authors’ knowledge no report demonstrating that the junctophilin MORN repeats directly interact with lipids [1, 12, 13, 90,91,92,93]

Summary

Introduction

MORN (Membrane Occupation and Recognition Nexus) repeats were first discovered in 2000, as a result of a screen for proteins present in the triad junctions of skeletal muscle [1]. The junctophilins, the protein family identified in this screen, were observed to have 8 repeats present in their N-terminal regions. The repeats were given the name MORN based on a proposed role in mediating plasma membrane association of the N-terminal domain of the junctophilins. A bioinformatics analysis at the time indicated that assemblies of 8 consecutive MORN repeats were present in a putative junctophilin orthologue in a nematode (Caenorhabditis elegans), a family of plant (Arabidopsis thaliana) lipid kinases, and a bacterial (Cyanobacterium) protein [1]. Later genome-era bioinformatics has shown that MORN repeat proteins are found ubiquitously, being present in both eukaryotes and prokaryotes [2]

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Results

Conclusion