Molecular Mechanisms of Intracellular Sorting and Transport of Membrane Proteins

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Mainly working on budding yeast, we obtained the following results regarding sorting mechanisms of membrane proteins.1)Study on Rerlp sorting receptor. We showed that Rerlp is the machinery that binds a subset of ER-resident membrane proteins when they are mistransported to the Golgi, by directly recognizing polar residue motifs in their TMD, and send them back to the ER.2)Study on lectin-type sorting receptors. We suggested that Emp46/47p act as receptors for a subset of glycoproteins during ER-Golgi recycling and showed that their oligomerization status is important for their trafficking.3)Complete reconstitution of membrane-cargo sorting during COPII budding. We reconstituted COPII vesicle budding in a cell-free system comprising only pure proteins and lipids in a cargo-dependent manner. By this assay, we demonstrated that GTP hydrolysis by Sarlp plays a proof-reading role in cargo selection.4)Roles of lipids and ubiquitin in membrane protein sorting. We showed that transport of plasma-membrane proteins, Tat2p and Pmalp, to the cell surface is regulated by ubiquitination and that sterol lipid plays an important role in this process.5)Real-time visualization of membrane-protein sorting. Using a high-speed and high-sensitivity confocal laser scanning microscope we developed, we demonstrated that intra-Golgi transport of cargo proteins is performed by the cisternal maturation mechanism. We further showed that dynamic sorting and segregation of membrane are involved in this process in COPI-dependent and independent ways.