title: Comprehensive Phylogenetic Analysis Sheds Light on the Diversity and Origin of the MLO Family of Integral Membrane Proteins.
abstract: Mildew resistanceLocusO(MLO) proteins are polytopic integral membrane proteins that have long been considered as plant-specific and being primarily involved in plant-powdery mildew interactions. However, research in the past decade has revealed that MLO proteins diverged into a family with several clades whose members are associated with different physiological processes. We provide a largely increased dataset of MLO amino acid sequences, comprising nearly all major land plant lineages. Based on this comprehensive dataset, we defined seven phylogenetic clades and reconstructed the likely evolution of the MLO family in embryophytes. We further identified several MLO peptide motifs that are either conserved in all MLO proteins or confined to one or several clades, supporting the notion that clade-specific diversification of MLO functions is associated with particular sequence motifs. In baker's yeast, some of these motifs are functionally linked to transmembrane (TM) transport of organic molecules and ions. In addition, we attempted to define the evolutionary origin of the MLO family and found that MLO-like proteins with highly diverse membrane topologies are present in green algae, but also in the distinctly related red algae (Rhodophyta), Amoebozoa, and Chromalveolata. Finally, we discovered several instances of putative fusion events between MLO proteins and different kinds of proteins. Such Rosetta stone-type hybrid proteins might be instructive for future analysis of potential MLO functions. Our findings suggest that MLO is an ancient protein that possibly evolved in unicellular photosynthetic eukaryotes, and consolidated in land plants with a conserved topology, comprising seven TM domains and an intrinsically unstructured C-terminus.
authors.affiliation: Unit of Plant Molecular Cell Biology, Institute for Biology I, RWTH Aachen University, 52056 Aachen, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Kusch
authors.affiliation: Unit of Plant Molecular Cell Biology, Institute for Biology I, RWTH Aachen University, 52056 Aachen, Germany.
authors.forename: Lina
authors.initials: L
authors.lastname: Pesch
authors.affiliation: Unit of Plant Molecular Cell Biology, Institute for Biology I, RWTH Aachen University, 52056 Aachen, Germany panstruga@bio1.rwth-aachen.de.
authors.forename: Ralph
authors.initials: R
authors.lastname: Panstruga
mesh_terms: D017360:Arabidopsis / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics; D000460:Chlorophyta / Q000235:genetics; D019143:Evolution, Molecular; D014644:Genetic Variation; D008565:Membrane Proteins / Q000235:genetics*; D005810:Multigene Family; D010788:Photosynthesis / Q000235:genetics; D010802:Phylogeny; D000461:Rhodophyta / Q000235:genetics
chemical_list: D029681:Arabidopsis Proteins; C573497:MLO2 protein, Arabidopsis; D008565:Membrane Proteins
keywords: MLO (Mildew resistance locus O); Rosetta stone protein; functional divergence; intrinsically disordered protein; polytopic integral membrane protein