Topology, Subcellular Localization, and Sequence Diversity of the Mlo Family in Plants

TLDR

Barley Mlo is a novel plant integral membrane protein whose loss confers broad‑spectrum powdery mildew resistance and deregulated leaf cell death, and its sequence diversity and topology resemble metazoan G‑protein‑coupled receptors. Mlo is a 7‑transmembrane protein anchored in the plasma membrane with an extracellular N‑terminus and intracellular C‑terminus, and Arabidopsis harbors approximately 35 such family members.

Abstract

Barley Mlo defines the founder of a novel class of plant integral membrane proteins. Lack of the wild type protein leads to broad spectrum disease resistance against the pathogenic powdery mildew fungus and deregulated leaf cell death. Scanning<i>N</i>-glycosylation mutagenesis and Mlo-Lep fusion proteins demonstrated that Mlo is membrane-anchored by 7 transmembrane (TM) helices such that the N terminus is located extracellularly and the C terminus intracellularly. Fractionation of leaf cells and immunoblotting localized the protein to the plant plasma membrane. A genome-wide search for <i>Mlo</i> sequence-related genes in<i>Arabidopsis thaliana</i> revealed approximately 35 family members, the only abundant gene family encoding 7 TM proteins in higher plants. The sequence variability of Mlo family members within a single species, their topology and subcellular localization are reminiscent of the most abundant class of metazoan 7 TM receptors, the G-protein-coupled receptors.