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9.B.197 The Mildew Resistance Locus O (MLO) Family 

Mildew resistance Locus O (MLO) proteins are polytopic integral membrane proteins that are plant-specific and primarily concerned with plant-powdery mildew interactions. They have a conserved topology with seven transmembrane domains and an intrinsically unstructured C-terminus. MLO proteins diverged into a family with several clades whose members are associated with different physiological processes. A dataset of MLO amino acyl sequences, comprising nearly all major land plant lineages, has been compiled (Kusch et al. 2016). Seven phylogenetic clades and several MLO peptide motifs that are either conserved in all MLO proteins or confined to one or several clades were identified. Thus, 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 transport of organic molecules and ions.  MLO-like proteins with highly diverse membrane topologies are present in green and red algae (Rhodophyta), Amoebozoa and Chromalveolata, as well as plants. Putative fusion events between MLO proteins and different kinds of proteins suggest specific MLO functions (Kusch et al. 2016).

MLO5 and MLO9 selectively recruit the Ca2+ channel CNGC18-containing vesicles to the plasma membrane through the R-SNARE proteins, VAMP721 and VAMP722 in trans mode. Meng et al. 2020 identified members of the conserved 7 TMS MLO family (expressed in the pollen tube) as tethering factors for Ca2+ channels, revealing a mechanism of molecular integration of extracellular ovular cues and selective exocytosis. This work sheds light on the general regulation of MLO proteins in cell responses to environmental stimuli (Meng et al. 2020 identified members of the conserved 7 TMS MLO family (expressed in the pollen tube) as tethering factors for Ca2+ channels, revealing a mechanism of molecular integration of extracellular ovular cues and selective exocytosis. This work sheds light on the general regulation of MLO proteins in cell responses to environmental stimuli (Meng et al. 2020).

References associated with 9.B.197 family:

Kusch, S., L. Pesch, and R. Panstruga. (2016). Comprehensive phylogenetic analysis sheds light on the diversity and origin of the MLO family of integral membrane proteins. Genome Biol Evol. [Epub: Ahead of Print] 26893454
Meng, J.G., L. Liang, P.F. Jia, Y.C. Wang, H.J. Li, and W.C. Yang. (2020). Integration of ovular signals and exocytosis of a Ca channel by MLOs in pollen tube guidance. Nat Plants 6: 143-153. 32055051