Chapter 2 Biogenesis and Sorting of Plasma Membrane Proteins

Ion pump sorting in polarized renal epithelial cells

The plasma membranes of renal epithelial cells are divided into distinct apical and basolateral domains, which contain different inventories of ion transport proteins. Without this polarity vectorial ion and fluid transport would not be possible. Little is known of the signals and mechanisms that renal epithelial cells use to establish and maintain polarized distributions of their ion transport proteins. Analysis of ion pump sorting reveals that multiple complex signals participate in determining and regulating these proteins' subcellular localizations.

Topology, subcellular localization, and sequence diversity of the Mlo family in plants

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 N-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 Mlo sequence-related genes in Arabidopsis thaliana 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.